fix: player state not being updated to completed (#1257)

This is regression introduced in 1.0.0.

In 0.2.0, this logic was:

      case 'audio.onComplete':
          player.state = PlayerState.COMPLETED;
          player._completionController.add(null);

So along with the completionController stream being update, the state was set to completed.
In 1.0.0 there is no such functionality:

      case 'audio.onComplete':
        emitComplete(playerId);

Since audioplayers_platform_interface package doesn't have a reference to player.state anymore because it's now in audioplayers package, my fix consisted of simply listening to this same stream in audioplayers package and updating the state.

packages/audioplayers/lib/src/audioplayer.dart

@@ -36,6 +36,8 @@ class AudioPlayer {
     _playerState = state;
   }
 
+  late StreamSubscription _onPlayerCompleteStreamSubscription;
+
   final StreamController<PlayerState> _playerStateController =
       StreamController<PlayerState>.broadcast();
 
@@ -89,7 +91,11 @@ class AudioPlayer {
   ReleaseMode get releaseMode => _releaseMode;
 
   /// Creates a new instance and assigns an unique id to it.
-  AudioPlayer({String? playerId}) : playerId = playerId ?? _uuid.v4();
+  AudioPlayer({String? playerId}) : playerId = playerId ?? _uuid.v4() {
+    _onPlayerCompleteStreamSubscription = onPlayerComplete.listen((_) {
+      state = PlayerState.completed;
+    });
+  }
 
   Future<void> play(
     Source source, {
@@ -266,10 +272,11 @@ class AudioPlayer {
     // First stop and release all native resources.
     await release();
 
-    final futures = <Future>[];
-    if (!_playerStateController.isClosed) {
-      futures.add(_playerStateController.close());
-    }
+    final futures = <Future>[
+      if (!_playerStateController.isClosed) _playerStateController.close(),
+      _onPlayerCompleteStreamSubscription.cancel()
+    ];
+
     await Future.wait<dynamic>(futures);
   }
 }