Add support to exempt the battery usage with visible FGS notifications

If an app has a FGS with visible notifications, its battery usage during
that would be excluded from the background battery usage accounting
for the background restrictions.

Bug: 203105544
Test: atest FrameworksMockingServicesTests:BackgroundRestrictionTest
Change-Id: I15545ce44170149cb0f7d1470542ae4656f64bb5