fix double ringer volume slider

On voice capable devices (phones) ringer and notification volume
is linked. Don't show the second slider until we can break the link.

Change-Id: I7161f88f048234e5ca1050d4ad87224476e37a1a
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>
Merge pull request #4 from AOKP/kitkat

fix double ringer volume slider