Adding explicit prefix so we don't have to have an exception in prefs

If we add an explicit prefix (similar to what the Gadgets extension
does), we won't have to have a special handler for checkmatrix options
in Preferences::filterDataForSubmit().

Depends on core change I4049b666

Change-Id: Ic8420b896bc013840eb3a32420a49f6719f30610
Merge "Adding explicit prefix so we don't have to have an exception in prefs"