Add a tracker on background current drains for each uid

On detecting excessive battery usage of a background uid, we may
move the background restriction of the apps running in this uid to
more restrictive levels.

Bug: 200326767
Test: atest FrameworksMockingServicesTests:BackgroundRestrictionTest
Change-Id: I6c2d41e44367a283d8aa9491be683018a80a810c