Make device "master volume" the max of all channels instead of the avg

When we use the avg, it's very easy to "boost" the "right speaker" over 100%.

Before this patch, if you set the left channel to 50% and leave the
right channel at 100% (via pavucontrol), the "master channel" will be
the average of 75%. If you then raise the "master channel" by 10%,
you raise the right channel to 110%.

Bug: 388553

Differential Revision: https://phabricator.kde.org/D9726