Inform ActivityManager of initially inactive sessions

When posting a new notification, we currently only inform
about already active sessions, but not when the session is
already inactive.

There is also no need to track explicitly which sessions
were already reported as active as we can just re-report
the active or inactive state as often as we want. This
simplifies some of the state tracking.

Bug: 385075120
Flag: com.android.media.flags.enable_notifying_activity_manager_with_media_session_status_change
Test: manually verified + atest ActivityManagerFgsDelegateTest
Change-Id: I96f207342403075c3e8459331e7360516be1849a