A2DP Audio Focus

Update A2dpSinkStateMachine to handle audio focus for notifications and
navigation. Simplified state machine to boolean flags to track user
intent and availability of audio stream and audio focus.

Bug: 31215324
Test: Manual music streaming, navigation, and assistant.  Tested
navigation, assistant and music streaming with phones from several major
manufacturers.  Tests were to verify proper transitions in both audio
and playback state while music was playing locally and streamed via
Bluetooth. Additionally b/34722788 was written to capture unit tests.

Change-Id: If8e89588ca24bdedf2c57c989f8a960353ccbe9a