Add background restriction levels definition and its controller

This CL adds the definition of background app restriction levels.

The existing various system background app restriction features will
be mapped into the levels here, including app standby buckets,
background restrictions (forced-app-standby) etc. Future CLs may
apply restrictions to background apps accordingly.

BYPASS_INCLUSIVE_LANGUAGE_REASON=Legacy API name

Bug: 200326767
Test: atest FrameworksMockingServicesTests:BackgroundRestrictionTest
Change-Id: Icf82031c572e7b8e82e6528402e9df4de5b7a675