Settings: Fix "Use actual storage type of sdcard/usb (1/2)."

Fixes http://review.cyanogenmod.org/63744

The original patch used the same string ids for sd in both cm_strings.xml and strings.xml.
Change them so there isn't a clash causing the wrong strings to be displayed by always prefixing the aosp ids with either sd_ or usb_.

Change-Id: Iad509123a9b4b93677b6ec06f15c34d9b0f57165