AudioDeviceVolumeManager and VolumeInfo API changes

Context service name for accessing AudioDeviceVolume manager.
Address API comments on VolumeInfo and AudioDeviceVolumeManager.
Add support in VolumeInfo to know whether it carries an explicit
  mute/unmute information (vs just default value of isMuted()).

Bug: 244326361
Test: atest android.media.audio.cts.AudioDeviceVolumeManagerTest
Test: atest android.media.audio.cts.VolumeInfoTest
Change-Id: Id718fbd7e0d5795ac480d9188faad4a70fba9808