Handle the bitmap loading result with applicationHandler

Before this change, the bitmap loading result with mainHandler, in which we set the metadata to `MediaSessionCompat`. However, the `MediaSessionCompat` is not thread safe, all calls should be made from the same thread. In the other calls to `MediaSessionCompat`, we ensure that they are on the application thread (which may be or may not be main thread), so we should do the same for `setMetadata` when bitmap arrives.

Also removes a comment in `DefaultMediaNotificationProvider` as bitmap request caching is already moved to CacheBitmapLoader.

PiperOrigin-RevId: 490524209

libraries/session/src/main/java/androidx/media3/session/DefaultMediaNotificationProvider.java

@@ -244,8 +244,6 @@ public interface NotificationIdProvider {
   private final String channelId;
   @StringRes private final int channelNameResourceId;
   private final NotificationManager notificationManager;
-  // Cache the last bitmap load request to avoid reloading the bitmap again, particularly useful
-  // when showing a notification for the same item (e.g. when switching from playing to paused).
   private final Handler mainHandler;
 
   private @MonotonicNonNull OnBitmapLoadedFutureCallback pendingOnBitmapLoadedFutureCallback;

libraries/session/src/main/java/androidx/media3/session/MediaSessionLegacyStub.java

@@ -118,7 +118,6 @@
   private final MediaPlayPauseKeyHandler mediaPlayPauseKeyHandler;
   private final MediaSessionCompat sessionCompat;
   @Nullable private VolumeProviderCompat volumeProviderCompat;
-  private final Handler mainHandler;
 
   private volatile long connectionTimeoutMs;
   @Nullable private FutureCallback<Bitmap> pendingBitmapLoadCallback;
@@ -162,7 +161,6 @@ public MediaSessionLegacyStub(
     @Initialized
     MediaSessionLegacyStub thisRef = this;
     sessionCompat.setCallback(thisRef, handler);
-    mainHandler = new Handler(Looper.getMainLooper());
   }
 
   /** Starts to receive commands. */
@@ -1205,7 +1203,9 @@ public void onFailure(Throwable t) {
                 }
               };
           Futures.addCallback(
-              bitmapFuture, pendingBitmapLoadCallback, /* executor= */ mainHandler::post);
+              bitmapFuture,
+              pendingBitmapLoadCallback,
+              /* executor= */ sessionImpl.getApplicationHandler()::post);
         }
       }
       setMetadata(