Accessiblity scroll event firing callback not properly reset.

1. We use a delayed callback to throttle the amount of accessibility
   scroll events fired by the view tree. The callback to do so was
   not properly reset when removed putting the view tree in a bad
   state resulting in no scroll events being fired at all.

bug:6549005

Change-Id: Ibf72d7e009e4545a336c9471f46015910290703e