Add utility objects to unify all media metadata types.

Right now there are four types of metadata in the media framework.
MediaMetadata, MediaDescripion, MediaSession.QueueItem, and
MediaBrowser.MediaItem. This patch provides utility functions to merge
all of these into the Metadata class to be used throughout AVRCP.

Bug: 68854188
Test: runtest bluetooth -c com.android.bluetooth.avrcp.MediaPlayerWrapperTest
Change-Id: I0213473b7a6e54e8a1ca7a3d651bf12ad6139f8e
(cherry picked from commit dbed4ea9ddc73fb248feb2e2f513752696ce4536)
Merged-In: I24b3816db2410dd7428f443036cbf869da1fb89e