AudioService: updates to death of focus requester

On the death of a focus requester that is in the focus stack:
- keep track of event in AudioService event log
- notify focus follower policies of the focus loss

Bug: 155098568
Test: "adb shell dumpsys audio | grep died" after killing a focus client
Test: atest com.google.android.gts.audio.AudioHostTest#testFocusChangeMediaToGuidance
Change-Id: I455ca4c5292c609421b5ed37176ff94e756dd00b