messageTray: Set the state to SHOWN when the notification is updated

If the notification is updated while SHOWING, we'll overwrite the
tween updating it to the new 'y' position, but forget to update the
state to SHOWN at the end of our transition. Make sure to always set
the state to SHOWN at the end.

https://bugzilla.gnome.org/show_bug.cgi?id=704844