MediaSession2: Add SessionCallback#onCurrentMediaItemChanged()

Added callback for session to know the currently playing media item has
changed.

Note that the callback is called in response to the
MediaPlayerBase#PlayerEventCallback#onCurrentDataSourceChanged(mpb, dsd
is called. Session will translate dsd to the media item and calls
onCurrentMediaItemChanged().

Following changes are also included
  - Removed MediaPlaylistController#getCurrentPlaylistItem(),
    because currently playing item is managed by the MediaPlayerBase.
  - Renamed ControllerCallback#onCurrentPlaylistItemChanged() to the
    ControllerCallback#onCurrentMediaItemChanged(), to make it more
    obvious that the event is from MediaPlayerBase, not
    MediaPlaylistController.
  - Added SessionCallback#onCurrentMediaItemChanged()

Bug: 64098437
Test: Run MediaComponents test
Change-Id: I78b124a7da0f968b097b2576507b9a73e36081ec