🐛 Fix connectivity issues

I'm fed up with all the extra effort for supporting an outdated
Android version. Apologies for the few users that are now excluded,
but I feel like it's just not worth the hassle.

app/build.gradle

@@ -21,7 +21,7 @@ android {
     }
     defaultConfig {
         applicationId "ch.dissem.apps.${appName.toLowerCase()}"
-        minSdkVersion 19
+        minSdkVersion 21
         targetSdkVersion 27
         versionCode 23
         versionName "1.0-rc1"

app/src/main/AndroidManifest.xml

@@ -175,20 +175,7 @@
 
         <!-- Receive Wi-Fi connection state changes -->
         <receiver
-            android:name=".listener.WifiReceiver"
-            android:enabled="@bool/is_pre_api_21">
-            <intent-filter>
-                <!-- This is bad for battery life, but needed on older devices to check
-                     if WiFi is available. Let's be honest, the whole app is bad for
-                     battery life. -->
-                <action
-                    android:name="android.net.conn.CONNECTIVITY_CHANGE"
-                    tools:ignore="BatteryLife" />
-            </intent-filter>
-        </receiver>
-        <receiver
-            android:name=".service.StartServiceReceiver"
-            android:enabled="@bool/is_post_api_21">
+            android:name=".service.StartServiceReceiver">
             <intent-filter>
                 <action android:name="android.intent.action.BOOT_COMPLETED" />
             </intent-filter>

app/src/main/java/ch/dissem/apps/abit/MainActivity.kt

@@ -179,6 +179,8 @@ class MainActivity : AppCompatActivity(), ListSelectionListener<Serializable> {
                 .setDelay(1000)
                 .show()
         }
+
+        SyncAdapter.startSync(this)
     }
 
     private fun <F> changeList(listFragment: F) where F : Fragment, F : ListHolder<*> {

app/src/main/java/ch/dissem/apps/abit/MessageDetailActivity.kt

@@ -4,7 +4,6 @@ import android.content.Intent
 import android.os.Bundle
 import android.support.v4.app.NavUtils
 import android.view.MenuItem
-import ch.dissem.bitmessage.entity.Conversation
 import ch.dissem.bitmessage.entity.Plaintext
 
 

app/src/main/java/ch/dissem/apps/abit/SettingsFragment.kt

@@ -18,8 +18,6 @@ package ch.dissem.apps.abit
 
 import android.app.Activity
 import android.content.*
-import android.os.Build
-import android.os.Build.VERSION_CODES.LOLLIPOP
 import android.os.Bundle
 import android.os.IBinder
 import android.support.v4.app.Fragment
@@ -191,21 +189,10 @@ class SettingsFragment : PreferenceFragmentCompat(), SharedPreferences.OnSharedP
 
     override fun onSharedPreferenceChanged(sharedPreferences: SharedPreferences, key: String) {
         when (key) {
-            PREFERENCE_TRUSTED_NODE -> toggleSyncTrustedNode(sharedPreferences)
             PREFERENCE_SERVER_POW -> toggleSyncServerPOW(sharedPreferences)
         }
     }
 
-    private fun toggleSyncTrustedNode(sharedPreferences: SharedPreferences) {
-        val node = sharedPreferences.getString(PREFERENCE_TRUSTED_NODE, null)
-        val ctx = context ?: throw IllegalStateException("No context available")
-        if (node != null) {
-            SyncAdapter.startSync(ctx)
-        } else {
-            SyncAdapter.stopSync(ctx)
-        }
-    }
-
     private fun toggleSyncServerPOW(sharedPreferences: SharedPreferences) {
         val node = sharedPreferences.getString(PREFERENCE_TRUSTED_NODE, null)
         if (node != null) {
@@ -264,7 +251,7 @@ class SettingsFragment : PreferenceFragmentCompat(), SharedPreferences.OnSharedP
     private fun connectivityChangeListener() =
         OnPreferenceChangeListener { _, _ ->
             context?.let { ctx ->
-                if (Build.VERSION.SDK_INT >= LOLLIPOP && Preferences.isFullNodeActive(ctx)) {
+                if (Preferences.isFullNodeActive(ctx)) {
                     NetworkUtils.scheduleNodeStart(ctx)
                 }
             }

app/src/main/java/ch/dissem/apps/abit/adapter/AndroidCryptography.kt

@@ -1,29 +0,0 @@
-/*
- * Copyright 2016 Christian Basler
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package ch.dissem.apps.abit.adapter
-
-import ch.dissem.apps.abit.util.PRNGFixes
-import ch.dissem.bitmessage.cryptography.sc.SpongyCryptography
-
-/**
- * @author Christian Basler
- */
-class AndroidCryptography : SpongyCryptography() {
-    init {
-        PRNGFixes.apply()
-    }
-}

app/src/main/java/ch/dissem/apps/abit/adapter/ConversationAdapter.kt

@@ -149,7 +149,7 @@ class ConversationAdapter internal constructor(
             linksClickable = true
             setTextIsSelectable(true)
         }
-        val labelAdapter = LabelAdapter(itemView.context, emptySet<Label>())
+        val labelAdapter = LabelAdapter(itemView.context, emptySet())
         val labels = itemView.findViewById<RecyclerView>(R.id.labels)!!.apply {
             adapter = labelAdapter
             layoutManager = GridLayoutManager(itemView.context, 2)

app/src/main/java/ch/dissem/apps/abit/adapter/LabelAdapter.kt

@@ -2,7 +2,6 @@ package ch.dissem.apps.abit.adapter
 
 import android.content.Context
 import android.content.res.ColorStateList
-import android.os.Build
 import android.support.annotation.ColorInt
 import android.support.v7.widget.RecyclerView
 import android.view.LayoutInflater
@@ -15,7 +14,6 @@ import ch.dissem.apps.abit.util.getIcon
 import ch.dissem.apps.abit.util.getText
 import ch.dissem.bitmessage.entity.valueobject.Label
 import com.mikepenz.iconics.view.IconicsImageView
-import org.jetbrains.anko.backgroundColor
 
 class LabelAdapter internal constructor(private val ctx: Context, labels: Collection<Label>) :
     RecyclerView.Adapter<LabelAdapter.ViewHolder>() {
@@ -50,11 +48,7 @@ class LabelAdapter internal constructor(private val ctx: Context, labels: Collec
         var label = itemView.findViewById<TextView>(R.id.label)!!
 
         fun setBackground(@ColorInt color: Int) {
-            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
             itemView.backgroundTintList = ColorStateList.valueOf(color)
-            } else {
-                itemView.backgroundColor = color
-            }
         }
     }
 }

app/src/main/java/ch/dissem/apps/abit/dialog/FullNodeDialogActivity.kt

@@ -17,7 +17,6 @@
 package ch.dissem.apps.abit.dialog
 
 import android.app.Activity
-import android.os.Build
 import android.os.Bundle
 import ch.dissem.apps.abit.R
 import ch.dissem.apps.abit.util.NetworkUtils
@@ -37,9 +36,7 @@ class FullNodeDialogActivity : Activity() {
             finish()
         }
         dismiss.setOnClickListener {
-            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
             NetworkUtils.scheduleNodeStart(applicationContext)
-            }
             finish()
         }
     }

app/src/main/java/ch/dissem/apps/abit/listener/WifiReceiver.kt

@@ -1,36 +0,0 @@
-/*
- * Copyright 2016 Christian Basler
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package ch.dissem.apps.abit.listener
-
-import android.content.BroadcastReceiver
-import android.content.Context
-import android.content.Intent
-import ch.dissem.apps.abit.service.Singleton
-import ch.dissem.apps.abit.util.NetworkUtils
-import ch.dissem.apps.abit.util.Preferences
-import org.jetbrains.anko.connectivityManager
-
-class WifiReceiver : BroadcastReceiver() {
-    override fun onReceive(ctx: Context, intent: Intent) {
-        if ("android.net.conn.CONNECTIVITY_CHANGE" == intent.action) {
-            val bmc = Singleton.getBitmessageContext(ctx)
-            if (Preferences.isFullNodeActive(ctx) && !bmc.isRunning() && !(Preferences.isWifiOnly(ctx) && ctx.connectivityManager.isActiveNetworkMetered)) {
-                NetworkUtils.doStartBitmessageService(ctx)
-            }
-        }
-    }
-}

app/src/main/java/ch/dissem/apps/abit/service/BitmessageService.kt

@@ -22,7 +22,6 @@ import android.content.Context
 import android.content.Intent
 import android.content.IntentFilter
 import android.net.ConnectivityManager
-import android.os.BatteryManager
 import android.os.Handler
 import ch.dissem.apps.abit.notification.NetworkNotification
 import ch.dissem.apps.abit.notification.NetworkNotification.Companion.NETWORK_NOTIFICATION_ID

app/src/main/java/ch/dissem/apps/abit/service/Singleton.kt

@@ -20,7 +20,6 @@ import android.content.Context
 import android.widget.Toast
 import ch.dissem.apps.abit.MainActivity
 import ch.dissem.apps.abit.R
-import ch.dissem.apps.abit.adapter.AndroidCryptography
 import ch.dissem.apps.abit.adapter.SwipeableMessageAdapter
 import ch.dissem.apps.abit.adapter.SwitchingProofOfWorkEngine
 import ch.dissem.apps.abit.listener.MessageListener
@@ -29,6 +28,7 @@ import ch.dissem.apps.abit.repository.*
 import ch.dissem.apps.abit.util.Constants
 import ch.dissem.apps.abit.util.Observable
 import ch.dissem.bitmessage.BitmessageContext
+import ch.dissem.bitmessage.cryptography.sc.SpongyCryptography
 import ch.dissem.bitmessage.entity.BitmessageAddress
 import ch.dissem.bitmessage.entity.payload.Pubkey
 import ch.dissem.bitmessage.entity.valueobject.Label
@@ -112,7 +112,7 @@ object Singleton {
                     ServerPowEngine(ctx),
                     ServicePowEngine(ctx)
                 )
-                cryptography = AndroidCryptography()
+                cryptography = SpongyCryptography()
                 nodeRegistry = AndroidNodeRegistry(sqlHelper)
                 inventory = AndroidInventory(sqlHelper)
                 addressRepo = AndroidAddressRepository(sqlHelper)

app/src/main/java/ch/dissem/apps/abit/service/StartupNodeOnWifiService.kt

@@ -14,8 +14,8 @@ import ch.dissem.apps.abit.util.Preferences
  *
  * And stops it when the preconditions for the job (unmetered network) aren't met anymore.
  */
-@RequiresApi(Build.VERSION_CODES.LOLLIPOP)
 class StartupNodeOnWifiService : JobService() {
+
     override fun onStartJob(params: JobParameters?): Boolean {
         val bmc = Singleton.getBitmessageContext(this)
         if (Preferences.isFullNodeActive(this) && !bmc.isRunning()) {
@@ -29,4 +29,5 @@ class StartupNodeOnWifiService : JobService() {
      * depending on Android version.
      */
     override fun onStopJob(params: JobParameters?) = Preferences.isFullNodeActive(this)
+
 }

app/src/main/java/ch/dissem/apps/abit/synchronization/SyncAdapter.kt

@@ -24,6 +24,7 @@ import ch.dissem.apps.abit.service.Singleton
 import ch.dissem.apps.abit.synchronization.Authenticator.Companion.ACCOUNT_POW
 import ch.dissem.apps.abit.synchronization.Authenticator.Companion.ACCOUNT_SYNC
 import ch.dissem.apps.abit.synchronization.StubProvider.Companion.AUTHORITY
+import ch.dissem.apps.abit.util.NetworkUtils
 import ch.dissem.apps.abit.util.Preferences
 import ch.dissem.bitmessage.exception.DecryptionFailedException
 import ch.dissem.bitmessage.extensions.CryptoCustomMessage
@@ -75,8 +76,8 @@ class SyncAdapter(context: Context, autoInitialize: Boolean) : AbstractThreadedS
         }
         val trustedNode = Preferences.getTrustedNode(context)
         if (trustedNode == null) {
-            LOG.info("Trusted node not available, disabling synchronization")
-            stopSync(context)
+            // As Abit tends to get killed by the system, let's leverage the sync mechanism to start it again:
+            NetworkUtils.scheduleNodeStart(context)
             return
         }
         LOG.info("Synchronization started")
@@ -104,7 +105,8 @@ class SyncAdapter(context: Context, autoInitialize: Boolean) : AbstractThreadedS
         // If the Bitmessage context acts as a full node, synchronization isn't necessary
         LOG.info("Looking for completed POW")
 
-        val privateKey = identity.privateKey?.privateEncryptionKey ?: throw IllegalStateException("Identity without private key")
+        val privateKey =
+            identity.privateKey?.privateEncryptionKey ?: throw IllegalStateException("Identity without private key")
         val signingKey = cryptography().createPublicKey(identity.publicDecryptionKey)
         val reader = ProofOfWorkRequest.Reader(identity)
         val powRepo = Singleton.getProofOfWorkRepository(context)
@@ -113,7 +115,8 @@ class SyncAdapter(context: Context, autoInitialize: Boolean) : AbstractThreadedS
             val (objectMessage, nonceTrialsPerByte, extraBytes) = powRepo.getItem(initialHash)
             val target = cryptography().getProofOfWorkTarget(objectMessage, nonceTrialsPerByte, extraBytes)
             val cryptoMsg = CryptoCustomMessage(
-                    ProofOfWorkRequest(identity, initialHash, CALCULATE, target))
+                ProofOfWorkRequest(identity, initialHash, CALCULATE, target)
+            )
             cryptoMsg.signAndEncrypt(identity, signingKey)
             val response = bmc.send(
                 trustedNode,

app/src/main/java/ch/dissem/apps/abit/util/Assets.kt

@@ -17,8 +17,6 @@
 package ch.dissem.apps.abit.util
 
 import android.content.Context
-import android.support.annotation.DrawableRes
-import android.support.annotation.StringRes
 import ch.dissem.apps.abit.R
 import ch.dissem.bitmessage.entity.Plaintext
 import java.io.IOException

app/src/main/java/ch/dissem/apps/abit/util/NetworkUtils.kt

@@ -6,10 +6,7 @@ import android.app.job.JobScheduler
 import android.content.ComponentName
 import android.content.Context
 import android.content.Intent
-import android.os.Build
-import android.support.annotation.RequiresApi
 import android.support.v4.content.ContextCompat
-import ch.dissem.apps.abit.MainActivity
 import ch.dissem.apps.abit.dialog.FullNodeDialogActivity
 import ch.dissem.apps.abit.service.BitmessageService
 import ch.dissem.apps.abit.service.StartupNodeOnWifiService
@@ -20,28 +17,10 @@ object NetworkUtils {
     fun enableNode(ctx: Context, ask: Boolean = true) {
         Preferences.setFullNodeActive(ctx, true)
 
-        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
         if (Preferences.isConnectionAllowed(ctx) || !ask) {
             scheduleNodeStart(ctx)
         } else {
-                askForConnection(ctx)
-            }
-        } else {
-            if (Preferences.isWifiOnly(ctx)) {
-                if (Preferences.isConnectionAllowed(ctx)) {
-                    doStartBitmessageService(ctx)
-                    MainActivity.updateNodeSwitch()
-                } else if (ask) {
-                    askForConnection(ctx)
-                }
-            } else {
-                doStartBitmessageService(ctx)
-                MainActivity.updateNodeSwitch()
-            }
-        }
-    }
-
-    private fun askForConnection(ctx: Context) {
+            // Ask for connection
             val dialogIntent = Intent(ctx, FullNodeDialogActivity::class.java)
             if (ctx !is Activity) {
                 dialogIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
@@ -49,6 +28,7 @@ object NetworkUtils {
             }
             ctx.startActivity(dialogIntent)
         }
+    }
 
     fun doStartBitmessageService(ctx: Context) {
         ContextCompat.startForegroundService(ctx, Intent(ctx, BitmessageService::class.java))
@@ -59,7 +39,6 @@ object NetworkUtils {
         ctx.stopService(Intent(ctx, BitmessageService::class.java))
     }
 
-    @RequiresApi(Build.VERSION_CODES.LOLLIPOP)
     fun scheduleNodeStart(ctx: Context) {
         val jobScheduler = ctx.getSystemService(Context.JOB_SCHEDULER_SERVICE) as JobScheduler
         val serviceComponent = ComponentName(ctx, StartupNodeOnWifiService::class.java)

app/src/main/java/ch/dissem/apps/abit/util/PRNGFixes.java

@@ -1,345 +0,0 @@
-/*
- * This software is provided 'as-is', without any express or implied
- * warranty.  In no event will Google be held liable for any damages
- * arising from the use of this software.
- *
- * Permission is granted to anyone to use this software for any purpose,
- * including commercial applications, and to alter it and redistribute it
- * freely, as long as the origin is not misrepresented.
- */
-
-package ch.dissem.apps.abit.util;
-
-import android.os.Build;
-import android.os.Process;
-import android.util.Log;
-
-import java.io.ByteArrayOutputStream;
-import java.io.DataInputStream;
-import java.io.DataOutputStream;
-import java.io.File;
-import java.io.FileInputStream;
-import java.io.FileOutputStream;
-import java.io.IOException;
-import java.io.OutputStream;
-import java.io.UnsupportedEncodingException;
-import java.security.NoSuchAlgorithmException;
-import java.security.Provider;
-import java.security.SecureRandom;
-import java.security.SecureRandomSpi;
-import java.security.Security;
-
-/**
- * Fixes for the output of the default PRNG having low entropy.
- * <p/>
- * The fixes need to be applied via {@link #apply()} before any use of Java
- * Cryptography Architecture primitives. A good place to invoke them is in the
- * application's {@code onCreate}.
- *
- * @see <a href="http://android-developers.blogspot.ch/2013/08/some-securerandom-thoughts.html">
- * http://android-developers.blogspot.ch/2013/08/some-securerandom-thoughts.html</a>
- */
-@SuppressWarnings("WeakerAccess")
-public final class PRNGFixes {
-
-    private static final int VERSION_CODE_JELLY_BEAN = 16;
-    private static final int VERSION_CODE_JELLY_BEAN_MR2 = 18;
-    private static final byte[] BUILD_FINGERPRINT_AND_DEVICE_SERIAL =
-        getBuildFingerprintAndDeviceSerial();
-
-    /**
-     * Hidden constructor to prevent instantiation.
-     */
-    private PRNGFixes() {
-    }
-
-    /**
-     * Applies all fixes.
-     *
-     * @throws SecurityException if a fix is needed but could not be applied.
-     */
-    public static void apply() {
-        applyOpenSSLFix();
-        installLinuxPRNGSecureRandom();
-    }
-
-    /**
-     * Applies the fix for OpenSSL PRNG having low entropy. Does nothing if the
-     * fix is not needed.
-     *
-     * @throws SecurityException if the fix is needed but could not be applied.
-     */
-    private static void applyOpenSSLFix() throws SecurityException {
-        if ((Build.VERSION.SDK_INT < VERSION_CODE_JELLY_BEAN)
-            || (Build.VERSION.SDK_INT > VERSION_CODE_JELLY_BEAN_MR2)) {
-            // No need to apply the fix
-            return;
-        }
-
-        try {
-            // Mix in the device- and invocation-specific seed.
-            Class.forName("org.apache.harmony.xnet.provider.jsse.NativeCrypto")
-                .getMethod("RAND_seed", byte[].class)
-                .invoke(null, (Object) generateSeed());
-
-            // Mix output of Linux PRNG into OpenSSL's PRNG
-            int bytesRead = (Integer) Class.forName(
-                "org.apache.harmony.xnet.provider.jsse.NativeCrypto")
-                .getMethod("RAND_load_file", String.class, long.class)
-                .invoke(null, "/dev/urandom", 1024);
-            if (bytesRead != 1024) {
-                throw new IOException(
-                    "Unexpected number of bytes read from Linux PRNG: "
-                        + bytesRead);
-            }
-        } catch (Exception e) {
-            throw new SecurityException("Failed to seed OpenSSL PRNG", e);
-        }
-    }
-
-    /**
-     * Installs a Linux PRNG-backed {@code SecureRandom} implementation as the
-     * default. Does nothing if the implementation is already the default or if
-     * there is not need to install the implementation.
-     *
-     * @throws SecurityException if the fix is needed but could not be applied.
-     */
-    private static void installLinuxPRNGSecureRandom()
-        throws SecurityException {
-        if (Build.VERSION.SDK_INT > VERSION_CODE_JELLY_BEAN_MR2) {
-            // No need to apply the fix
-            return;
-        }
-
-        // Install a Linux PRNG-based SecureRandom implementation as the
-        // default, if not yet installed.
-        Provider[] secureRandomProviders =
-            Security.getProviders("SecureRandom.SHA1PRNG");
-        if ((secureRandomProviders == null)
-            || (secureRandomProviders.length < 1)
-            || (!LinuxPRNGSecureRandomProvider.class.equals(
-            secureRandomProviders[0].getClass()))) {
-            Security.insertProviderAt(new LinuxPRNGSecureRandomProvider(), 1);
-        }
-
-        // Assert that new SecureRandom() and
-        // SecureRandom.getInstance("SHA1PRNG") return a SecureRandom backed
-        // by the Linux PRNG-based SecureRandom implementation.
-        SecureRandom rng1 = new SecureRandom();
-        if (!LinuxPRNGSecureRandomProvider.class.equals(
-            rng1.getProvider().getClass())) {
-            throw new SecurityException(
-                "new SecureRandom() backed by wrong Provider: "
-                    + rng1.getProvider().getClass());
-        }
-
-        SecureRandom rng2;
-        try {
-            rng2 = SecureRandom.getInstance("SHA1PRNG");
-        } catch (NoSuchAlgorithmException e) {
-            throw new SecurityException("SHA1PRNG not available", e);
-        }
-        if (!LinuxPRNGSecureRandomProvider.class.equals(
-            rng2.getProvider().getClass())) {
-            throw new SecurityException(
-                "SecureRandom.getInstance(\"SHA1PRNG\") backed by wrong"
-                    + " Provider: " + rng2.getProvider().getClass());
-        }
-    }
-
-    /**
-     * {@code Provider} of {@code SecureRandom} engines which pass through
-     * all requests to the Linux PRNG.
-     */
-    private static class LinuxPRNGSecureRandomProvider extends Provider {
-
-        LinuxPRNGSecureRandomProvider() {
-            super("LinuxPRNG",
-                1.0,
-                "A Linux-specific random number provider that uses"
-                    + " /dev/urandom");
-            // Although /dev/urandom is not a SHA-1 PRNG, some apps
-            // explicitly request a SHA1PRNG SecureRandom and we thus need to
-            // prevent them from getting the default implementation whose output
-            // may have low entropy.
-            put("SecureRandom.SHA1PRNG", LinuxPRNGSecureRandom.class.getName());
-            put("SecureRandom.SHA1PRNG ImplementedIn", "Software");
-        }
-    }
-
-    /**
-     * {@link SecureRandomSpi} which passes all requests to the Linux PRNG
-     * ({@code /dev/urandom}).
-     */
-    @SuppressWarnings("JavaDoc")
-    public static class LinuxPRNGSecureRandom extends SecureRandomSpi {
-
-        /*
-         * IMPLEMENTATION NOTE: Requests to generate bytes and to mix in a seed
-         * are passed through to the Linux PRNG (/dev/urandom). Instances of
-         * this class seed themselves by mixing in the current time, PID, UID,
-         * build fingerprint, and hardware serial number (where available) into
-         * Linux PRNG.
-         *
-         * Concurrency: Read requests to the underlying Linux PRNG are
-         * serialized (on sLock) to ensure that multiple threads do not get
-         * duplicated PRNG output.
-         */
-
-        private static final File URANDOM_FILE = new File("/dev/urandom");
-
-        private static final Object sLock = new Object();
-
-        /**
-         * Input stream for reading from Linux PRNG or {@code null} if not yet
-         * opened.
-         *
-         * @GuardedBy("sLock")
-         */
-        private static DataInputStream sUrandomIn;
-
-        /**
-         * Output stream for writing to Linux PRNG or {@code null} if not yet
-         * opened.
-         *
-         * @GuardedBy("sLock")
-         */
-        private static OutputStream sUrandomOut;
-
-        /**
-         * Whether this engine instance has been seeded. This is needed because
-         * each instance needs to seed itself if the client does not explicitly
-         * seed it.
-         */
-        private boolean mSeeded;
-
-        @Override
-        protected void engineSetSeed(byte[] bytes) {
-            try {
-                OutputStream out;
-                synchronized (sLock) {
-                    out = getUrandomOutputStream();
-                }
-                out.write(bytes);
-                out.flush();
-            } catch (IOException e) {
-                // On a small fraction of devices /dev/urandom is not writable.
-                // Log and ignore.
-                Log.w(PRNGFixes.class.getSimpleName(),
-                    "Failed to mix seed into " + URANDOM_FILE);
-            } finally {
-                mSeeded = true;
-            }
-        }
-
-        @Override
-        protected void engineNextBytes(byte[] bytes) {
-            if (!mSeeded) {
-                // Mix in the device- and invocation-specific seed.
-                engineSetSeed(generateSeed());
-            }
-
-            try {
-                DataInputStream in;
-                synchronized (sLock) {
-                    in = getUrandomInputStream();
-                }
-                //noinspection SynchronizationOnLocalVariableOrMethodParameter
-                synchronized (in) {
-                    in.readFully(bytes);
-                }
-            } catch (IOException e) {
-                throw new SecurityException(
-                    "Failed to read from " + URANDOM_FILE, e);
-            }
-        }
-
-        @Override
-        protected byte[] engineGenerateSeed(int size) {
-            byte[] seed = new byte[size];
-            engineNextBytes(seed);
-            return seed;
-        }
-
-        private DataInputStream getUrandomInputStream() {
-            synchronized (sLock) {
-                if (sUrandomIn == null) {
-                    // NOTE: Consider inserting a BufferedInputStream between
-                    // DataInputStream and FileInputStream if you need higher
-                    // PRNG output performance and can live with future PRNG
-                    // output being pulled into this process prematurely.
-                    try {
-                        sUrandomIn = new DataInputStream(
-                            new FileInputStream(URANDOM_FILE));
-                    } catch (IOException e) {
-                        throw new SecurityException("Failed to open "
-                            + URANDOM_FILE + " for reading", e);
-                    }
-                }
-                return sUrandomIn;
-            }
-        }
-
-        private OutputStream getUrandomOutputStream() throws IOException {
-            synchronized (sLock) {
-                if (sUrandomOut == null) {
-                    sUrandomOut = new FileOutputStream(URANDOM_FILE);
-                }
-                return sUrandomOut;
-            }
-        }
-    }
-
-    /**
-     * Generates a device- and invocation-specific seed to be mixed into the
-     * Linux PRNG.
-     */
-    private static byte[] generateSeed() {
-        try {
-            ByteArrayOutputStream seedBuffer = new ByteArrayOutputStream();
-            DataOutputStream seedBufferOut =
-                new DataOutputStream(seedBuffer);
-            seedBufferOut.writeLong(System.currentTimeMillis());
-            seedBufferOut.writeLong(System.nanoTime());
-            seedBufferOut.writeInt(Process.myPid());
-            seedBufferOut.writeInt(Process.myUid());
-            seedBufferOut.write(BUILD_FINGERPRINT_AND_DEVICE_SERIAL);
-            seedBufferOut.close();
-            return seedBuffer.toByteArray();
-        } catch (IOException e) {
-            throw new SecurityException("Failed to generate seed", e);
-        }
-    }
-
-    /**
-     * Gets the hardware serial number of this device.
-     *
-     * @return serial number or {@code null} if not available.
-     */
-    private static String getDeviceSerialNumber() {
-        // We're using the Reflection API because Build.SERIAL is only available
-        // since API Level 9 (Gingerbread, Android 2.3).
-        try {
-            return (String) Build.class.getField("SERIAL").get(null);
-        } catch (Exception ignored) {
-            return null;
-        }
-    }
-
-    private static byte[] getBuildFingerprintAndDeviceSerial() {
-        StringBuilder result = new StringBuilder();
-        String fingerprint = Build.FINGERPRINT;
-        if (fingerprint != null) {
-            result.append(fingerprint);
-        }
-        String serial = getDeviceSerialNumber();
-        if (serial != null) {
-            result.append(serial);
-        }
-        try {
-            return result.toString().getBytes("UTF-8");
-        } catch (UnsupportedEncodingException e) {
-            throw new RuntimeException("UTF-8 encoding not supported");
-        }
-    }
-}

app/src/main/res/drawable-v21/bg_swipe_item_left.xml

@@ -1,25 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<!--
-       Copyright (C) 2015 Haruki Hasegawa
-
-       Licensed under the Apache License, Version 2.0 (the "License");
-       you may not use this file except in compliance with the License.
-       You may obtain a copy of the License at
-
-           http://www.apache.org/licenses/LICENSE-2.0
-
-       Unless required by applicable law or agreed to in writing, software
-       distributed under the License is distributed on an "AS IS" BASIS,
-       WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-       See the License for the specific language governing permissions and
-       limitations under the License.
--->
-<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
-    <item>
-        <color android:color="@color/bg_swipe_item_trash"/>
-    </item>
-    <item
-        android:drawable="@drawable/ic_item_swipe_trash"
-        android:gravity="right|center_vertical"
-        android:right="16dp"/>
-</layer-list>

app/src/main/res/drawable-v21/bg_swipe_item_right.xml

@@ -1,25 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<!--
-       Copyright (C) 2015 Haruki Hasegawa
-
-       Licensed under the Apache License, Version 2.0 (the "License");
-       you may not use this file except in compliance with the License.
-       You may obtain a copy of the License at
-
-           http://www.apache.org/licenses/LICENSE-2.0
-
-       Unless required by applicable law or agreed to in writing, software
-       distributed under the License is distributed on an "AS IS" BASIS,
-       WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-       See the License for the specific language governing permissions and
-       limitations under the License.
--->
-<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
-    <item>
-        <color android:color="@color/bg_swipe_item_archive"/>
-    </item>
-    <item
-        android:drawable="@drawable/ic_item_swipe_archive"
-        android:gravity="left|center_vertical"
-        android:left="16dp"/>
-</layer-list>

app/src/main/res/drawable-v21/material_showcase_button_bg.xml

@@ -1,9 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<ripple xmlns:android="http://schemas.android.com/apk/res/android"
-        android:color="#ffffff">
-
-    <item
-            android:id="@android:id/mask"
-            android:drawable="@android:color/white"/>
-
-</ripple>

app/src/main/res/drawable/bg_swipe_item_left.xml

@@ -18,9 +18,8 @@
     <item>
         <color android:color="@color/bg_swipe_item_trash"/>
     </item>
-    <item android:right="16dp">
-        <bitmap
+    <item
+        android:drawable="@drawable/ic_item_swipe_trash"
         android:gravity="right|center_vertical"
-            android:src="@drawable/ic_item_swipe_trash"/>
-    </item>
+        android:right="16dp"/>
 </layer-list>

app/src/main/res/drawable/bg_swipe_item_right.xml

@@ -18,9 +18,8 @@
     <item>
         <color android:color="@color/bg_swipe_item_archive"/>
     </item>
-    <item android:left="16dp">
-        <bitmap
+    <item
+        android:drawable="@drawable/ic_item_swipe_archive"
         android:gravity="left|center_vertical"
-            android:src="@drawable/ic_item_swipe_archive"/>
-    </item>
+        android:left="16dp"/>
 </layer-list>

app/src/main/res/drawable/material_showcase_button_bg.xml

@@ -1,14 +1,9 @@
 <?xml version="1.0" encoding="utf-8"?>
-<selector xmlns:android="http://schemas.android.com/apk/res/android">
-    <item android:state_pressed="true">
-        <shape android:shape="rectangle">
-            <solid android:color="#ccffffff" />
-        </shape>
-    </item>
-    <item android:state_focused="true">
-        <shape android:shape="rectangle">
-            <stroke android:color="@android:color/white" />
-        </shape>
-    </item>
-    <item android:drawable="@android:color/transparent" />
-</selector>
+<ripple xmlns:android="http://schemas.android.com/apk/res/android"
+        android:color="#ffffff">
+
+    <item
+            android:id="@android:id/mask"
+            android:drawable="@android:color/white"/>
+
+</ripple>

app/src/main/res/layout/showcase_button.xml

@@ -1,11 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<Button xmlns:android="http://schemas.android.com/apk/res/android"
-        android:layout_width="wrap_content"
-        android:layout_height="wrap_content"
-        android:background="@drawable/material_showcase_button_bg"
-        android:text="@string/got_it"
-        android:textAllCaps="true"
-        android:textColor="@android:color/white"
-        android:textSize="13sp"
-        android:textStyle="bold">
-</Button>

app/src/main/res/values-v21/attrs.xml

@@ -1,5 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<resources>
-    <bool name="is_pre_api_21">false</bool>
-    <bool name="is_post_api_21">true</bool>
-</resources>

app/src/main/res/values/attrs.xml

@@ -1,5 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<resources>
-    <bool name="is_pre_api_21">true</bool>
-    <bool name="is_post_api_21">false</bool>
-</resources>

app/src/main/res/xml/preferences.xml

@@ -34,7 +34,6 @@
         <SwitchPreferenceCompat
             android:defaultValue="false"
             android:key="require_charging"
-            android:enabled="@bool/is_post_api_21"
             android:summary="@string/require_charging_summary"
             android:title="@string/require_charging" />
         <SwitchPreferenceCompat