Don't prompt on bg abuse if the app has a FGS with notification

Add a new config to control whether or not to prompt the user when
the system detects a background abusive app and it has a foreground
service with notification shown. By default it's OFF, meaning if
the background abusive app has a foreground service and its
notification is still showing, no prompt to the user in this case.

Bug: 200326767
Bug: 203105544
Test: atest FrameworksMockingServicesTests:BackgroundRestrictionTest
Change-Id: Ic611090e3152117a2379d902fc32416e11c3e197