volume: Ignore slider changes we initiated ourselves

Commit 21e14bd4 fixed this for the brightness slider, but we have the same problem for volume too. When the volume is muted - for example in Settings or via a media key, we update the slider to '0' to indicate this visually. But we also actually invoke the slider's callback to set the volume to zero. That means that the previous level is overwritten so it can't be restored when unmuting.

The fix is the same - when we update the slider internally ourselves, don't call the signal handler.