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

Conflicts:
	policy/src/com/android/internal/policy/impl/PhoneWindowManager.java