kiosk-shell: Don't occlude shsurf on other outputs

This adds an additional check to make sure the current focus surface
is on the same output as the surface that is going to be activated.

This is necessary in order to avoid placing the currently focused one in
the inactive layer, which shouldn't happen in situations where the new
surface is going to be placed on a different output than the currently
focused one.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>