No default background set for GNOME-Greeter

[Matthew Hagemann]

While using Gnome-Initial-Setup launched from GDM (when no user > 1000 is present on the machine), no background is set on GNOME-Greeter, leading to a solid blue background wallpaper.

Could we set the background here to the Ubuntu default? By updating /usr/share/glib-2.0/schemas/10_ubuntu-settings.gschema.override with:

```
# for GDM/DM
# FIXME: move to :Ubuntu-Greeter once upstream supports this, see LP: #1788

[org.gnome.desktop.interface:GNOME-Greeter]
gtk-theme = 'Yaru'
icon-theme = 'Yaru'
cursor-theme = 'Yaru'
font-name = 'Ubuntu 11'
monospace-font-name = 'Ubuntu Mono 13'
font-antialiasing = 'rgba'

[org.gnome.login-screen]
logo='/usr/share/plymouth/ubuntu-logo.png'

+ [org.gnome.desktop.background:GNOME-Greeter]
+ picture-uri = 'file:///usr/share/backgrounds/warty-final-ubuntu.png'
+ picture-uri-dark = 'file:///usr/share/backgrounds/ubuntu-wallpaper-d.png'
+ show-desktop-icons = false
```

I was able to add in one. This will help improve the GIS installer flow for Raspberry Pi we plan to support in 24.10

ubuntu-settings:
  Installed: 24.04.3
Ubuntu version: 24.04 LTS