toggle play/pause on cammera button longpress while screen is off

this is a patch to add the ability to toggle play/pause on camera long press
while the screen is off, on those devices that have hardware camera buttons.

if music is not playing when the screen is initially turned off, the camera button
will have no effect, this is to prevent the button from turning music on "accidentally"
and to ensure it can only be used to resume music that was initially paused using
this functionality in the first place.

this patch also sets up options to enable/disable this functionality as well
as the volume button music controls (volbtn controls enabled by default,
cambtn controls disabled by default).  there are related commits in CMParts

Change-Id: Ibc7bafd0775bbe8f59fa8ac734ba4d7d764cfab8