Add CarVolumeDialogController in SystemUI for Android Auto.

Cars usually have an external audio module. When Android is serving as
the car's headunit, users should be able to adjust the car's volume
through SystemUI. The following changes are made to make it work:

+ Load VolumeDialogController from SystemUIFactory
+ Added CarSystemUIFactory
+ Added CarVolumeDialogController which extends VolumeDialogController
  and it uses CarAudioManager as source of truth for volume controls.
+ Some refactor in VolumeDialogController to make it easier for
subclasses to override volume controls.

Note that CarAudioManager does not completely replace AudioManager.
Majority of code in VolumeDialogController still applies in the car use
case, so I made CarVolumeDialogController a subclass of
VolumeDialogController instead of making them peers.

Bug: 27595951

Change-Id: Id4adec7281e41aa71f3de034e5b88a32a89be305