Relax bg->fgs restriction for system initiated transitions.

For system initiated bg->fgs transition, we need to relax restrictions
to allow for usecases where system decides to put service in fgs.

Test: atest CtsAppFgsTestCases
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerNotifyMediaFGSTypeTest.java;
Flag: com.android.media.flags.enable_notifying_activity_manager_with_media_session_status_change
Bug: 383481936
Bug: 281762171

Change-Id: I072d874250d646ffd9a77d335743df988e53a256
Signed-off-by: Pradeep Sawlani <sawlani@google.com>