New intent for microphone mute change notification

AudioManager: define ACTION_MICROPHONE_MUTE_CHANGED.
AudioService:
 - broadcast intent when setMicrophoneMute() is called
   with FLAG_RECEIVER_REGISTERED_ONLY.
 - remove outdated (invalid) comments.

Bug: 70857126
Test: m -j ; make -j offline-sdk-docs ; make -j cts ; \
      cts-tradefed run cts-dev -m CtsMediaTestCases -t android.media.cts.AudioManagerTest#testMicrophoneMuteIntent

Change-Id: I7391522f0195d06592e4f853e9b672fdec66d37a