-Added media session implementation for all players.

-Extracted version numbers in gradle dependencies.
-Updated ExoPlayer to 2.7.1.
-Updated RxJava to 2.1.10, RxAndroid to 2.0.2 and RxBinding to 2.1.1.
-Removed deprecated implementation of media buttons.

app/src/main/AndroidManifest.xml

@@ -43,12 +43,6 @@
             android:launchMode="singleTask"
             android:label="@string/title_activity_background_player"/>
 
-        <receiver android:name="org.schabi.newpipe.player.BackgroundPlayer$MediaButtonReceiver">
-            <intent-filter>
-                <action android:name="android.intent.action.MEDIA_BUTTON" />
-            </intent-filter>
-        </receiver>
-
         <activity
             android:name=".player.PopupVideoPlayerActivity"
             android:launchMode="singleTask"