Fix for, BT Carkit position issue when music app is killed

When Music application is killed by user or lower memory killer,
Playback state is not set to null (or to default).
So, BT AVRCP display (or remote display) continues to display with
last music song position.

Bug: 134560226
Test: 1. Connect to BT carkit
Test: 2. Play music with music apk
Test: 3. Force stop music apk in phone settings->app->force stop
Test: 4. Start navigation with map apk.
Test:  Manuallytested the above scenerio on two carkits.

Change-Id: I84a8c245f4c2fc258a316f498e2737942d438af6