kiosk-shell: Check if app_ids have been set after initial commit

Some applications would set-up the app_id after the initial commit
(without a buffer) which is too late to correctly assign the application
to the corresponding output set-up in the configuration file.

This patch fixes that by checking one more time, after a buffer has been
attached, if indeed there's an output with an app_id set.

Fixes: #469

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