alsa-sink/source: Make sure volumes are synchronised after fast user switching

alsa-sink/source: Make sure volumes are synchronised after fast user switching

Log in as user A, fast user switch to user B, let user B change
port, volume or mute status, then switch back to user A.

At this point we must make sure that the ALSA and PA volumes are
synchronised by writing to the ALSA mixer when the ALSA device
becomes available.

BugLink: https://bugs.launchpad.net/bugs/915035
Signed-off-by: David Henningsson <david.henningsson@canonical.com>