AVRCP Controller request metadata on playback.

Track metadata is only provided when requested, and the current
implementation only requested it when there was a track change
notification.  Add a request for metadata whenever there is an
update to indicate that the play status is playing.

Bug: 63998350
Test: Start playing media, make a phone call, end a phonecall observe
metadata is correct.

Change-Id: Iea5cdacfe95c1a98cdaf73836ee6359a923f481b