daemon-system: Allow list g-r-d user for GDM
Previously GDM shipped d-bus policy to allow gnome-remote-desktop to talk to it. This caused two problems:
- Errors from the bus daemon on systems that don't have g-r-d installed
- Hardcoding the username "gnome-remote-desktop" when it's configurable on the g-r-d side
This commit adds the necessary policy to the g-r-d side instead.