Decouple the thresholds of the background restrictions

Previously, to reach the threshold of the bg restricted level, it has
to reach the threshold of the restricted bucket level before that.
However we're using different dimensions of battery usage when
comparing to these thresholds.

So it'll make more sense to decouple them.

Bug: 203105544
Test: atest FrameworksMockingServicesTests:BackgroundRestrictionTest
Change-Id: Idee31d6615cbbee0bac2abe232589346d838a9ba