GitLab

Moved VolumeControl from backend/alsa to backend directory. To remove
ALSA dependency added SystemVolumeControl interface that abstracts
functionaly implemented in AlsaVolumeControl.

Bug: 772488
Change-Id: I40575d873df00829b445dc8309ad0a61038009c8
Reviewed-on: https://chromium-review.googlesource.com/720076
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510014}