add option to force high-end graphics on low memory devices

Change-Id: I34e0d7381615485b58385abed743e01f7272772d

Conflicts:
	res/values/cm_strings.xml
	src/com/android/settings/cyanogenmod/PerformanceSettings.java

Conflicts:
	res/values/cm_strings.xml
	src/com/android/settings/cyanogenmod/PerformanceSettings.java