Make setGenericVolume keep balance between channels

Previously setGenericVolume would apply the same amount of difference
on all channels, making e.g. 100%/50% -> 80%/30%.

This commit changes it to keep the ratios equal instead, so that the
resulting volume would be 80%/40%, keeping the balance the same.

BUG: 435840
FIXED-IN: 5.27.7


(cherry picked from commit cfe4a360)