Settings: Fix NPE in QS on tablets

On tablets, onCreate is only called once, while onActivityCreated is called every
time the fragment is visible. So a second remove of a preferent will result in a NPE

Change-Id: I30205380018f29bd99270c707bd4f8ba03103648
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>

Conflicts:
	src/com/android/settings/cyanogenmod/QuickSettings.java