AudioPolicy: focus stack query, send focus loss APIs

Add APIs in android.media.audiopolicy.AudioPolicy to:
- query the full focus stack
- send a full audio focus loss to a stack entry so it loses
  focus and exits the stack.

Bug: 186582089
Test: atest AudioHostTest#testFocusStack
Change-Id: I28cba1daf3bcabd50d6192573ecd8b9ddae03c8e