Add handling of simultaneous alarms/timers

Initial implementation handled only one alarm at a time, if a new alarm came in, a notification for it would not be shown until the initial alarm is muted/dismissed.

Now we can keep track of all uids that have audio focus for ALARM usage type and therefore handle each of them independently.

Bug: 367615180
Test: atest BackgroundUserSoundNotifier
Flag: android.multiuser.multiple_alarm_notifications_support
Change-Id: I7a988e6c8f426b596ebc93f1433174424e62b50f