Avoid acquiring lock when feature flag is disabled

As of now, the flag doesn't prevent the acquisition of
MediaSessionService#mLock in some methods, which is causing
deadlocks.

Bug: 335454915
Flag: com.android.media.flags.enable_notifying_activity_manager_with_media_session_status_change
Test: Presubmit
Change-Id: I5b16c7f6b5c4996ff40e50274c0e968a3f355674