Added a restart song button to signle track expanded notification

... and some comments to the code

app/src/main/java/org/schabi/newpipe/player/BackgroundPlayer.java

@@ -219,20 +219,30 @@ public final class BackgroundPlayer extends Service {
         remoteViews.setOnClickPendingIntent(R.id.notificationContent,
                 PendingIntent.getActivity(this, NOTIFICATION_ID, intent, PendingIntent.FLAG_UPDATE_CURRENT));
 
-        if (basePlayerImpl.playQueue != null && basePlayerImpl.playQueue.size() > 1) {
+        // Lets check if we are playing more than one song in the background
+        if (basePlayerImpl.playQueue != null && basePlayerImpl.playQueue.size() > 1) { // If we have more than one song
+            // Use track skiping for forward and back
             remoteViews.setInt(R.id.notificationFRewind, SET_IMAGE_RESOURCE_METHOD, R.drawable.exo_controls_previous);
             remoteViews.setInt(R.id.notificationFForward, SET_IMAGE_RESOURCE_METHOD, R.drawable.exo_controls_next);
             remoteViews.setOnClickPendingIntent(R.id.notificationFRewind,
                     PendingIntent.getBroadcast(this, NOTIFICATION_ID, new Intent(ACTION_PLAY_PREVIOUS), PendingIntent.FLAG_UPDATE_CURRENT));
             remoteViews.setOnClickPendingIntent(R.id.notificationFForward,
                     PendingIntent.getBroadcast(this, NOTIFICATION_ID, new Intent(ACTION_PLAY_NEXT), PendingIntent.FLAG_UPDATE_CURRENT));
-        } else {
+            // We dont need a restart track button (skip track backwards)
+            remoteViews.setViewVisibility(R.id.notificationRestartTrack, View.INVISIBLE);
+        } else { // But if we only have one song
+            // Use time skipping for fastforward/rewind
             remoteViews.setInt(R.id.notificationFRewind, SET_IMAGE_RESOURCE_METHOD, R.drawable.exo_controls_rewind);
             remoteViews.setInt(R.id.notificationFForward, SET_IMAGE_RESOURCE_METHOD, R.drawable.exo_controls_fastforward);
             remoteViews.setOnClickPendingIntent(R.id.notificationFRewind,
                     PendingIntent.getBroadcast(this, NOTIFICATION_ID, new Intent(ACTION_FAST_REWIND), PendingIntent.FLAG_UPDATE_CURRENT));
             remoteViews.setOnClickPendingIntent(R.id.notificationFForward,
                     PendingIntent.getBroadcast(this, NOTIFICATION_ID, new Intent(ACTION_FAST_FORWARD), PendingIntent.FLAG_UPDATE_CURRENT));
+            // Add a restart track button (skip track backwards)
+            remoteViews.setViewVisibility(R.id.notificationRestartTrack, View.VISIBLE);
+            remoteViews.setOnClickPendingIntent(R.id.notificationRestartTrack,
+                    PendingIntent.getBroadcast(this, NOTIFICATION_ID, new Intent(ACTION_PLAY_PREVIOUS), PendingIntent.FLAG_UPDATE_CURRENT));
+
         }
 
         setRepeatModeIcon(remoteViews, basePlayerImpl.getRepeatMode());

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

@@ -116,6 +116,22 @@
             android:src="@drawable/ic_repeat_white"
             tools:ignore="ContentDescription"/>
 
+        <ImageButton
+            android:id="@+id/notificationRestartTrack"
+            android:layout_width="40dp"
+            android:layout_height="match_parent"
+            android:layout_centerVertical="true"
+            android:layout_marginRight="5dp"
+            android:layout_toLeftOf="@+id/notificationFRewind"
+            android:background="#00000000"
+            android:clickable="true"
+            android:focusable="true"
+            android:padding="2dp"
+            android:scaleType="fitCenter"
+            android:src="@drawable/exo_controls_previous"
+            android:visibility="gone"
+            tools:ignore="ContentDescription"/>
+
         <ImageButton
             android:id="@+id/notificationFRewind"
             android:layout_width="40dp"