kiosk-shell: Handle child top-level windows at destruction

With the seperation of surface activation and keyboard input, a special
corner case arose for child top-level windows when surfaces are being
destroyed. To make sure we never pick the wrong window to activate upon
destruction, we verify if the current focused surface is different than
the one being destroyed.

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