AudioService: focus abandon notifies focus followers

When a focus owner abandons focus, notify focus
followers.

Bug: 155098568
Test: atest com.google.android.gts.audio.AudioHostTest#testFocusChangeMediaToGuidance
Test: atest com.google.android.gts.audio.AudioHostTest#testFocusAbandonNotification
Change-Id: I5b05663cd526a28c8f537bbd8760ace4e461f168