AudioService: fix missing CEC message on unmute after vol down

  Group in one method sending volume / mute updates to HDMI CEC
when applicable. Use this method, maybeSendSystemAudioStatusCommand,
to avoid code duplication.
  Add missing synchronization in onUnmuteStream for stream mute check.
  When STREAM_MUSIC is unmuted, also call
maybeSendSystemAudioStatusCommand for a mute adjust, to ensure
other CEC devices' UI is in sync with the internal state.

Bug: 136977965
Test: with ATV device in system audio mode, mute with remote, then \
  press vol down on TV remote, verify TV display matches ATV UI    \
  with current volume.

Change-Id: I279a97871ba98e67f282bf148969ba41ed3f6062