client-info: Check for `app-flatpak-` prefix in `get_xdg_id`

Seems to be the correct prefix to check for as of https://github.com/flatpak/flatpak/commit/0c291cf1c9251a93a7bb893ee8e0371e588e05dc. (Which follows a change in https://github.com/flatpak/flatpak/commit/e481e3ea58e515e6e88673fac908be941c882569.)

Seems to fix recognition of Flatpak apps on Pop!_OS 21.04 beta. Otherwise Flatpak apps are recognized as "system apps", and are always authorized without invoking the agent.