CRAS a2dp: Add softvolume to a2dp device
CRAS a2dp: Add softvolume to a2dp device

BUG=chromium:220380
TEST=gmerge adhd, connect to a2dp device, can use volume key
to adjust volume.

Change-Id: Ic9a07187e03a155ba8c152b9344bf7cdf9f271cc
Reviewed-on: https://gerrit.chromium.org/gerrit/49300
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Commit-Queue: Hsinyu Chao <8b91762bdbe27d3121b311091d7ca5125e9a8df6@chromium.org>
Tested-by: Hsinyu Chao <8b91762bdbe27d3121b311091d7ca5125e9a8df6@chromium.org>

drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd