audio: Add volume alias for PC with single volume

This volume alias is used to separate a11y volume from the media volume
when config_single_volume is on.

Bug: 376307941
Test: Enable talkback, make sure there is a separate a11y volume slider
in the volume dialog
Flag: EXEMPT desktop only

Change-Id: Ie2da23cbbd415e395760f86f8f1f553c80985100