Make the volume feedback implementation common to all backends

Using a new singleton class VolumeFeedback, which receives volume
change events via a ControlManager announcement and plays the sound
for a volume change on the current global master device.  This also
handles rate limiting and stopping at maximum volume as per the
referenced bugs.  Remove the corresponding code from the PulseAudio
and ALSA backends.

In theory this means that volume feedback will also work with OSS,
assuming that Canberra supports OSS output.

CCBUG:302361
CCBUG:302474
CCBUG:323608