messageTray: Hide summary notification immediately when closing the tray

When the summary notification is open when the tray is closed, we end
up with two concurrent animations: the notification fading out, and the
tray moving away from underneath it. Sliding out the tray should be the
primary transition here, so hide the notification immediately to not
draw the user's attention away from it.

https://bugzilla.gnome.org/show_bug.cgi?id=686888