convert PlayerHolder to Singleton, handle context within, bugfix ServiceConnection leak

- bugfix: have ServiceConnection created only once!

- select the context within the PlayerHolder to start, stop, bind or unbind the service
  -> we have to make sure the Service is started AND stopped within the same context
  -> so let PlayerHolder be the one to select the context

- remove removeListener() and replace the call with setListener(null)
- Compatibility: use ContextCompat.startForegroundService instead of startService()

app/src/main/java/org/schabi/newpipe/util/NavigationHelper.java

@@ -349,13 +349,13 @@ public final class NavigationHelper {
                                                final boolean switchingPlayers) {
 
         final boolean autoPlay;
-        @Nullable final MainPlayer.PlayerType playerType = PlayerHolder.getType();
+        @Nullable final MainPlayer.PlayerType playerType = PlayerHolder.getInstance().getType();
         if (playerType == null) {
             // no player open
             autoPlay = PlayerHelper.isAutoplayAllowedByUser(context);
         } else if (switchingPlayers) {
             // switching player to main player
-            autoPlay = PlayerHolder.isPlaying(); // keep play/pause state
+            autoPlay = PlayerHolder.getInstance().isPlaying(); // keep play/pause state
         } else if (playerType == MainPlayer.PlayerType.VIDEO) {
             // opening new stream while already playing in main player
             autoPlay = PlayerHelper.isAutoplayAllowedByUser(context);

app/src/main/java/org/schabi/newpipe/util/StreamDialogEntry.java

@@ -39,7 +39,7 @@ public enum StreamDialogEntry {
      * Info: Add this entry within showStreamDialog.
      */
     enqueue(R.string.enqueue_stream, (fragment, item) -> {
-        final MainPlayer.PlayerType type = PlayerHolder.getType();
+        final MainPlayer.PlayerType type = PlayerHolder.getInstance().getType();
 
         if (type == AUDIO) {
             NavigationHelper.enqueueOnBackgroundPlayer(fragment.getContext(),