Cleaned up the permission checks in NetworkScoreService

Refactored the existing code into 3 permission check methods
to clear up what was being checked and to centralize the logic.

Bug:64313251
Test: bit services FrameworksServicesTests:com.android.server.NetworkScoreServiceTest
Test: bit FrameworksWifiTests:*
Test: gts-tradefed run gts -m GtsGmscoreHostTestCases -t com.google.android.gts.netrec.NetRecHostTest
Change-Id: I77457ce7cf98252e83f4dfa6602da0179d638818