vss-agl: Add audio control signals

vss-agl: Add audio control signals

Add Vehicle.Cabin.Infotainment.Media.Audio.* signals for balance,
fade, treble, and bass controls to our overlay.  The plan is to
work towards pushing these into upstream VSS.

Bug-AGL: SPEC-5001

Change-Id: Ic779ea01936f2fa8b4d5cb918634d676bec53505
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29533
Tested-by: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account