Spotify: Update auto-muting code

- Updates the method used to detect when to mute.
- Remove tracking mute state - this is a quick hack until I daemonise
  i3companion, so that running 'watch -n 1 ~/.i3/i3companion/spotify.py'
  will be enough to do the auto-muting. The disadvantage is that it will
  force unmuting Spotify regardless of who muted it - as I said, it's a
  hack.