Simplify notification tracking in MediaSessionService

We are only really interested in new notifications for a
foreground service, so can ignore all non-FGS notifications
and also all follow-up notification updates for the same
notification. This should be a no-op change (no test failures)
and just reduces the number of calls through to ActivityManager.

Flag: com.android.media.flags.enable_notifying_activity_manager_with_media_session_status_change
Test: atest ActivityManagerNotifyMediaFGSTypeTest
Bug: 281762171
Change-Id: I3351f412edd48fcd523aa131f727b02ffdf536f2