Stay bound to the most current active scorer.

This fixes the following issue:

1) Scorer A is active and bound.
2) A change occurs to Scorer A such that it is no longer the selected
   scorer.
3) Scorer B is selected as the scorer and returned whenever
   getActiveScorer() is queried.
4) Scorer A is never unbound because it isn't seen as the active
   scorer.
5) Scorer B is never bound until its package changes in some way (or
   the feature is disabled/enabled).

Test: runtest frameworks-services -c com.android.server.NetworkScoreServiceTest + manual testing.
Change-Id: I40f055df472e2b9e7b07e6979a841dd4c0b794ce
Merged-In: I468a8c02a72f089f1fb778d150d945efe5485b88