background: Support dark wallpapers

Use the picture-uri-dark setting introduced in GNOME 42 to read the filename of the wallpaper when the color-scheme is prefer-dark. This follows the logic implemented in gnome-shell.

Use the same background settings for the screensaver wallpaper to make it consistent. The picture-uri key of the org.gnome.desktop.screensaver GSettings schema was not updated to reflect the recent changes in gnome-backgrounds, so the default setting causes blank background, because adwaita-timed.xml was removed from gnome-backgrounds. Since this setting is actually not used by gnome-shell, I think it's better to read the org.gnome.desktop.background GSettings schema instead.