Notification power widget mobiledata auto 2g/3g

Adds an option for the notification power widget mobiledata button to
automatically switch to GSM/2G only when mobile data is disabled and
WCDMA/3G pref when mobile data is enabled.
Saves battery/screen estate/manual effort for users who only turn
mobiledata+3G on when needed.

Second part: http://review.cyanogenmod.com/#change,9447

Change-Id: Id3e68a75b00b3fa63a3eaf0eebf119f5b31ebcd2