ospack,images: Make demopack available on OSTree images

The default demopack installs media files in `/home/shared`: by
including them only in the ospack they are unavailble on the ostree
images as the whole `/home` gets filtered out during `ostree commit`.

Moving the extraction of the demopack when the full filesystem gets
deployed to the images makes the media files available on ostree images
as well.

See https://phabricator.apertis.org/T5313



Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>