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.

Change-Id: Id5ffe1be8acf1579f93eab8ee1797d14000b9964