AudioPolicy: add API to make a focus owner lose focus

Add a new AudioPolicy method that allows a registered policy
to force a focus owner lose focus. If it took focus from another
focus stack entry, that one will now regain focus.

Flag: android.multiuser.add_ui_for_sounds_from_background_users
Bug: 355596432
Bug: 355576058
Test: atest com.android.server.audio.MediaFocusControlTest#testSendFocusLossAndUpdate
Change-Id: Ib739f95c0f2792ec9ead59862f52cd4e11d33594