Fix bug 2183179 - Hitting the back key repeatedly makes the notification shade jump up and down

This thing needs to be rewritten.  It's grown too complicated for what it's doing,
and I don't like the physics anyway.

Also, this leaves the bug where if you press back while it's animating down, the
animation is ignored.  However it no longer jumps down all the way, it just
essentially ignores the back key.

Also, this makes the collapse get executed on the back key up event, so it works
with the swipe on the soft keys at the bottom of the screen.