Add permission checks to getActiveScorerPackage().

Added and documented checks for REQUEST_NETWORK_SCORES
and SCORE_NETWORKS.

Bug: 62347124
Test: runtest frameworks-services -c com.android.server.NetworkScoreServiceTest
Test: gts-tradefed run gts -m GtsGmscoreHostTestCases -t com.google.android.gts.netrec.NetRecHostTest
Change-Id: Ibeba7079b3d4a75e11aa0ede6ad2be1c7d48324f