Fix notification panel bug which prevented proper resizing

When the notification panel is opened larger than the list's length,
the penl is put into a "rubber band" mode, to animate the bottom of the
panel back to the end of the list. But the rubber band mode is never
unset, which causes an improper result if another notification is ended,
preventing the panel from resizing to the new, appropriate size of the list.

Fix is to unset the rubber banding flag when the snapping animation finishes.

Issue #6992133 Notification bar does not drop down completely in Mako

Change-Id: Id93918086353f58d6eb6bb071556f08eb2e2706e