AudioService: fix volume initialization on first ever boot

When HDMI is connected, it calls checkFixedVolumeDevices() that
  iterates over the devices in the volume index map to set them
  at max index. But since the map only contains DEVICE_OUT_DEFAULT
  after a factory reset, VolumeStreamState.checkFixedVolumeDevices()
  doesn't do anything for DEVICE_OUT_HDMI because it is not in the map.
The fix consists in adding the new device in the volume map for a
  connection, and initializing its volume with the default volume,
  VolumeStreamState.checkFixedVolumeDevices() will in turn update
  the volume to max if needed (reads mask or full/fix vol devices).

Bug: 130766100
Test: factory reset device and start playback, check HDMI volume with dumpsys audio
Change-Id: I4c338df6acc1e15af9606694cdbd485d1b991bb5