Fix bug 2239212 "Dial pad has no sound"

The DTMF stream volume is tied to the in-call volume. In the dialer
outside of a call, the user has no way to change the touch tone
volume. If (s)he lowered the incall volume all the way down,
the user will usually complain that the dial pad has no sound,
and if they maxed it out, they'll complain it's too loud.
 This change causes the tones to use the media volume, outside of
a call only, and to change the media volume when the user presses
the hardware volume keys.
This gives us both the advantage of keeping the incall behavior consistent
with the existing behavior (AFAIK there are no complaints regarding
in call DTMF volume), but offers the user control over the volume
in the dialer before/after a call.