Fix AVRCP crash, position and state tracking

When the MediaController goes away (media stops) a NPE would cause a
crash.

Initialize the position so that an initial notification gets sent
when position is requested, send it when the music is paused, and
send play state whenever the app sends us a play state update.

Bug: 30039744
Bug: 29416450
Change-Id: I44397b3dc40d328fa75a2c15875fad298a5090dd