Hide summary box pointer without animating if it is empty

Hide summary box pointer without animating if it is empty

This ensures that we don't show a small black blob fading away
when the user clicks on the notification and it is removed.

Set HIDING state right away in _hideSummaryBoxPointer() so that
it is only called once. Update this._pointerInTray when the tray
is unlocked, so that we are not dependent on escapeTray() being
triggered by 'done-displaying-content' signal.

https://bugzilla.gnome.org/show_bug.cgi?id=645697