Audio focus: API for external audio focus policy

System API for an external audio focus policy. Extends
 the system focus listener with interception of
 focus requests and abandons. Adds method for the focus
 policy to dispatch focus grants and losses.

Test: gts-tradefed run gts -m GtsGmscoreHostTestCases -t 'com.google.android.gts.audio.AudioHostTest#testFocusPolicy'
Bug: 30258418
Change-Id: If408569a2dce07a774e0e2f1be9f1af8e426d2d3