The AC doesn't get a D-Bus path until it's exported, but that happens after
it's handed to the Device it will be activated on.  The Device emits a
PropertyChanged event when it's handed the AC, but it ignores ACs that
aren't exported yet.  Thus when activating, the Device doesn't emit the
AC's path at all in the ActiveConnection property because it's NULL.

Fix that by exporting the AC immediately before starting activation
with it.

Second, move the notification of the Device.ActiveConnection property
to be emitted along with the state change to PREPARE instead of long
before it.  While we don't guarantee signal ordering in general, this
seems like a more correct ordering.

https://bugzilla.gnome.org/show_bug.cgi?id=723783