keyboard: Don't set the XKB group switching option when not needed

We might set up different layouts in different groups - see
replace_layout_and_variant(). But we don't want the X server group
switching feature to actually switch them. Regularly, if we have at
least two input sources, gnome-shell will tell us to switch input
source at that point so we fix that automatically. But when there's
only one source, gnome-shell short circuits as an optimization and
doesn't call us so we can't set the group switching XKB option in the
first place otherwise the X server's switch will take effect and we
get a broken configuration.

https://bugzilla.gnome.org/show_bug.cgi?id=709085