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
