Added a onRestrictBackgroundBlacklistChanged() callback.

When an UID is added / removed to the Data Saver blacklist, it's
necessary to notify internal components such as the Settings UI (which
was erroneously listening to UID rules changes instead).

BUG: 28743623
BUG: 28791717

Change-Id: I11c85e141dfe074ad390fd324309d2412bfbbd45