Remove using hidden API: MediaSessionManager(Context)

Replace with SystemServiceRegistry.registerContextAwareService,
which is a @SystemApi. See ag/9734296 for reference.

Bug: 160380016
Test: atest
    CtsMediaTestCases:android.media.cts.MediaSessionManagerTest
Change-Id: I9b6da97e0e0f5181ce8681a399ca090e67f6fd4e
2 files changed