Add force usage and category for System Audio Control.

System Audio Control in HDMI-CEC delegate audio control
to audio receiver. When system audio control is selected
in audio settings as audio output it should select one of ARC, SPDIF,
and LINE as audio output.
Whenever user select System Audio as audio output, audio
setting should call
setForceUse(AUDIO_POLICY_FORCE_FOR_SYSTEM_AUDIO, AUDIO_POLICY_FORCE_XXX);
and if other output is selected it should call
setForceUse(AUDIO_POLICY_FORCE_FOR_SYSTEM_AUDIO, AUDIO_POLICY_FORCE_NONE);

Change-Id: I1c237ee9291a4e0acf01a86aff3176edce5c14e0