Remove power widget background

Change-Id: I314642782525fc441a651cc2b48b227f663dfeed
Port "Bind VolUp+VolDown to toggle the ringer"

Squash of http://review.cyanogenmod.org/#/c/20256/ and
http://review.cyanogenmod.org/#/c/22716/

Change-Id: Ied3767969482a7ea44f7f0e9222a2f78fe8f507c
AudioService: Don't adjust volume when vol-downing to vibrate or silent

Pressing vol-down to vibrate or silent would (previously) still adjust the
stream volume, setting the last audible volume to 0, which is not allowed
for voice-capable devices.  Thus, the last audible volume would reset to
default on reboot if on vibrate/silent or the lowest volume setting.

By not adjusting volume when vol-downing to vibrate or silent on
voice-capable devices, the last audible volume is always preserved across
reboot.