Handle package changes in NetworkScoreService.

Keep a registered receiver for changes to the active scorer package
when scoring is enabled. Disable scoring if any of those changes cause
the active scorer to become invalidated.

Covers the package or subcomponent being disabled, the package being
upgraded to a version that is no longer a valid scorer, or the package
being uninstalled. Intentionally keeps scoring active if the package
is upgraded and the new package is still a valid scorer.

Bug: 18494565
Change-Id: I1d340820b12881ab2ee147a788773df9cc7005d7