endSessionDialog: Initialize Polkit permission asynchronously

Found using sysprof gjs profiling, I haven't really tested if this breaks anything so far but it seems like this._updatesPermission is only used after checking for NULL before anyway.

The other big offender during shell startup is gweather_location_get_world, it seems like this needs to be converted to an async function though, so I opened an issue for that: libgweather#29 (closed)

---

Commit message:

The updatesPermission is currently initialized synchronously, which blocks the Mainloop for quite some time and therefore slows down startup of the shell, let's do it asynchronously instead.