Fix miniplayer disappearing when switching episode during cast (#8582)

### Description

Fix miniplayer disappearing when switching episode during cast. The
player comes back after a while (probably because of other service
changes since that issue was opened) but it disappearing is the wrong
behavior. With this change, the player stays active until actually
transitioning to "playing".

Closes #8504

### Checklist
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playback/cast/README.md

@@ -1,4 +1,18 @@
 # :playback:cast
 
 This module provides Chromecast support for the Google Play version of the app.
-Implements `PlaybackServiceMediaPlayer` using the Cast SDK; compiled only in the `play` build flavor.
+Compiled only in the `play` build flavor (the `free` flavor contains no-op stubs).
+
+## Active path (Media3)
+
+`CastPlayerWrapper.wrap(localPlayer, context)` builds a Media3 `CastPlayer` that automatically
+routes playback to the Cast device when a session is active and falls back to the local player
+otherwise. It uses `ApMediaItemConverter` (inner class) to translate between Media3 `MediaItem`
+and Cast `MediaQueueItem`. The media database ID is round-tripped through Cast metadata under
+the key `"media_id"`, which `Media3PlaybackService.ensureCurrentMediaLoaded()` reads back to
+look up the `FeedMedia` from the local database.
+
+## Legacy path (CastPsmp)
+
+`CastPsmp` implements `PlaybackServiceMediaPlayer` for the old `PlaybackService`. It is still
+compiled but is effectively dead code — `PlaybackService` throws an exception if started.

playback/service/src/main/java/de/danoeh/antennapod/playback/service/Media3PlaybackService.java

@@ -310,8 +310,6 @@ public class Media3PlaybackService extends MediaLibraryService {
         public void onMediaItemTransition(@Nullable MediaItem mediaItem, int reason) {
             if (mediaItem == null) {
                 currentPlayable = null;
-                PlaybackPreferences.writeNoMediaPlaying();
-                EventBus.getDefault().post(new PlayerStatusEvent());
             } else {
                 ensureCurrentMediaLoaded();
             }