Player/handleIntent: always early return on ENQUEUE an ENQUEUE_NEXT

We can do this, because:

1. if `playQueue` is not null, we return early
2. if `playQueue` is null and we need to enqueue:
  - the only “proper” case that could be triggered is
    the `RESUME_PLAYBACK` case, which is never `true` for the queuing
    intents, see the comment in `NavigationHelper.enqueueOnPlayer`
  - the generic `else` case is degenerate, because it would crash on
  `playQueue` being `null`.

This makes some sense, because there is no way to trigger the
enqueueing logic via the UI currently if there is no video playing
yet, in which case `playQueue` is not `null`.

So we need to transform this whole if desaster into a big switch.

Author: Profpatsch

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

@@ -368,15 +368,20 @@ public void handleIntent(@NonNull final Intent intent) {
         }
 
         // Resolve enqueue intents
-        if (intent.getBooleanExtra(ENQUEUE, false) && playQueue != null) {
-            playQueue.append(newQueue.getStreams());
+        if (intent.getBooleanExtra(ENQUEUE, false)) {
+            if (playQueue != null) {
+                playQueue.append(newQueue.getStreams());
+            }
             return;
+        }
 
         // Resolve enqueue next intents
-        } else if (intent.getBooleanExtra(ENQUEUE_NEXT, false) && playQueue != null) {
-            final int currentIndex = playQueue.getIndex();
-            playQueue.append(newQueue.getStreams());
-            playQueue.move(playQueue.size() - 1, currentIndex + 1);
+        if (intent.getBooleanExtra(ENQUEUE_NEXT, false)) {
+            if (playQueue != null) {
+                final int currentIndex = playQueue.getIndex();
+                playQueue.append(newQueue.getStreams());
+                playQueue.move(playQueue.size() - 1, currentIndex + 1);
+            }
             return;
         }