Fix crash in clearWallpaper()

The underlying clearWallpaper() service method demands that only a
single kind of wallpaper be specified as the target; but a recent
patch attempted to expand the client-side legacy method to apply to
all kinds of wallpaper, incorrectly.  This patch corrects that client-
side code to do things properly.

Bug 30456015

Change-Id: I0a881957b881206e5eb775c6879ba90f10f9ffb0