Restore @VisibleForTesting on NetworkScorerAppManager.

The NetworkScoreService tests fail (Mockito doesn't work) without
those methods being public :/

Also removed a failing test from NetworkScoreServiceTest since a
recent CL changed the behavior of dump (no longer throws).

Test: runtest frameworks-services -c com.android.server.NetworkScoreServiceTest
Change-Id: Id3fb9451027db3685b16b659a00a6a52b7a1e523