AVRCP Controller Play Position Changed

Implement code to register for the AVRCP notification
EVENT_PLAYBACK_POS_CHANGED such that playback position gets update on
skip to beginning, and during audio scrubbing from phone.

Bug: 119119967
Test: Play music, scrub audio to another position on phone, observe
playback position is in sync.

Change-Id: Ib8459dece7629622649ba6ed340dcd697c700b09