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/local/feed/FeedFragment.kt

@@ -331,7 +331,7 @@ class FeedFragment : BaseStateFragment<FeedState>() {
         if (context == null || context.resources == null || activity == null) return
 
         val entries = ArrayList<StreamDialogEntry>()
-        if (PlayerHolder.getType() != null) {
+        if (PlayerHolder.getInstance().getType() != null) {
             entries.add(StreamDialogEntry.enqueue)
         }
         if (item.streamType == StreamType.AUDIO_STREAM) {

app/src/main/java/org/schabi/newpipe/local/history/StatisticsPlaylistFragment.java

@@ -340,7 +340,7 @@ public class StatisticsPlaylistFragment
 
         final ArrayList<StreamDialogEntry> entries = new ArrayList<>();
 
-        if (PlayerHolder.getType() != null) {
+        if (PlayerHolder.getInstance().getType() != null) {
             entries.add(StreamDialogEntry.enqueue);
         }
         if (infoItem.getStreamType() == StreamType.AUDIO_STREAM) {

app/src/main/java/org/schabi/newpipe/local/playlist/LocalPlaylistFragment.java

@@ -749,7 +749,7 @@ public class LocalPlaylistFragment extends BaseLocalListFragment<List<PlaylistSt
 
         final ArrayList<StreamDialogEntry> entries = new ArrayList<>();
 
-        if (PlayerHolder.getType() != null) {
+        if (PlayerHolder.getInstance().getType() != null) {
             entries.add(StreamDialogEntry.enqueue);
         }
         if (infoItem.getStreamType() == StreamType.AUDIO_STREAM) {