Removing firing of spurious scroll accesibility events.

1. Adding a TextView to a layout fires an accessibility scroll event
   with mScrollX = 8144. Now TextView does not fire scroll events
   since they are not interesting for accessibility and also the
   implementation fires scroll to frequently - though correctly.

2. AbsListView was firing accessibility events for the same location.
   Here the caveat is that this class does not know its height and
   calls onScrollChange every time is scroll as seen by the user
   may have changed but for accessibility purposes we care for the
   start and end indices of visible views. The fix is to avoid
   sending access events with duplicate data.

bug:5097467

Change-Id: I7bf11efd638ea3673843f1095a03f518e57cfe9d