Remove media players if their backing session dissappears

If the backing session dies or dissappears for any reason, then remove
it from the tracked sessions. If it was the active session, send an
update that the media information has changed so that the remote device
doesn't continue attempting to retrieve information from the now defunct
session.

Bug: 125672467
Test: Kill the active music app while playing a song
Change-Id: Ia40c2a1ef962057ac0a3f824f084afbb031ec480