MediaSessionService: Clear calling identity for incoming calls from apps

This only clears calling identity if MediaSessionRecord calls
APIs of MediaSessionService.

Bug: 36682559
Test: Run CtsMediaTest (MediaSessionTest and MediaSessionManagerTest)
  and Manual test (Checked that a media app that changes the active
  state and supports cast doesn't crash)
Change-Id: Ia4a441b692ea7da606b1bafbe9a6c3f8fd690518