Followup I4049b666: "Removing prefix exception for HTMLCheckMatrix"

Change I4049b666 changed the way Preferences combines the prefix with
the row and column for HTMLCheckMatrix, but did not adjust the
corresponding code in User::getOptionFromUser().

Bug: 69146
Change-Id: I69ed9c875443ba44d1a5e12e51165ebf8d6d6a06
Merge "Followup I4049b666: "Removing prefix exception for HTMLCheckMatrix""