Merge changes from topics "playerinterface_onError",...

Merge changes from topics "playerinterface_onError", "mediasession2_notifychildren", "mediasession2_subscribe", "MediaSession2_rating_style_for_each_item"

* changes:
  MediaSession2: Add a way to notify errors between session and player
  MediaSession2: Implement MediaLibrarySession#notifyChildrenChanged()
  MediaSession2: Implement subscribe/unsubscribe
  MediaSession2: Remove rating style from session