Use dark style in greeter session

[lenemter]

Complementary to elementary/greeter#684

[davidmhewitt]

This probably isn't going to work in the expected way, or at all.

The Granite.Settings object first queries the Portal, which probably isn't running in the greeter. Then it queries AccountsService, which is running, but Granite queries using the current user (which is lightdm in the greeter session).

There's also a chance of hangs here because a lot of these DBus calls happen synchronously.

I would suggest that instead of this, the Greeter needs to communicate the dark style preference to the panel, rather than the panel process trying to detect it itself in the Greeter.

[lenemter]

@davidmhewitt elementary/greeter#684 both launches portals in the greeter session and syncs dark style preference from selected user's AccountsService to lightdm's one.