shell-extensions: Properly disable when not running Shell

Don't try to activate org.gnome.Shell. If we're not running Shell, it's
not going to be activatable, and we're showing an in-app error to tell
the user about this.

Instead we should not try to activate it, but just disable the
extensions plugin.

https://bugs.launchpad.net/ubuntu/+source/gnome-software/+bug/1713009