Fix a deadlock resulting in no UI being shown on first run

After switching to DBus activation, we need to ensure that a controller is present when arbitrary GApplication actions are executed so a main window also exists. This lead to a deadlock on first run since when the application was activated, it would be in the middle of constructing the controller when it showed the Accounts Editor, which would block since it would also try to construct a controller.

This fixes the deadlock by moving the first-run check to the application, outside of the lock.