portal: Own org.gnome.Shell.Portal for the access portal

org.freedesktop.impl.portal.desktop.gnome will be used for the
GNOME implementation of desktop portals in the future, so make
sure GNOME Shell's implementation of the access portal won't
conflict.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1872>
(cherry picked from commit 40543bdd)