Take AudioFocusStack state into account when showing/dismissing notification

Audio can lose focus due to being overtaken by another another focus request. In this case it remains in the focus stack but is moved from the top. If this happens, the notification should remain visible and only dismissed if the AudioFocusInfo leaves the stack. And if notification is visible, do not create create a new notification when the focus moves back to the top of the stack.

Refactor BackgroundUserSoundNotifier to improve readability.

Add new test cases.

Bug: 365971142
Bug: 355596432
Bug: 355576058
Test: atest BackgroundUserSoundNotifierTest
Flag: EXEMPT bugfix
Change-Id: Ide8be8219cefad660b62cd0e33023be9a9da6016