xdg_app_dir_get_installed_size: Implement without using ostree sizes
xdg_app_dir_get_installed_size: Implement without using ostree sizes

This is slightly slower, but actually works since we stopped
collecting sizes in the commits. Further work will make this more performant.

GeorgesStavracas/flatpak,matthiasclasen/flatpak,alexlarsson/xdg-app,matthiasclasen/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,handsome-feng/flatpak,alexlarsson/xdg-app,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,flatpak/flatpak,handsome-feng/flatpak,flatpak/flatpak,amigadave/flatpak,amigadave/flatpak,flatpak/flatpak,amigadave/flatpak,amigadave/flatpak,alexlarsson/xdg-app,GeorgesStavracas/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,alexlarsson/xdg-app