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