Stub APIs for network scoring.

NetworkScoreManager defines all interactions between the framework and
the scorer app. An ACTION_SCORE_NETWORKS broadcast is sent to the
default scorer with any unscored networks, which responds by calling
updateScores() with scores for those networks. An app may also check
whether they are currently the default scorer with
getDefaultScorerPackage() and request to become the default via the
ACTION_CHANGE_DEFAULT broadcast.

Bug: 13786258
Change-Id: Id3dc0f7c1109f0e3afd73356e2475b7f34167419