View's visibility to the user not checking predecessor alpha.

1. A view is visible to the user if is attached to a visible window,
   its visibility is VISIBLE, its alpha is not zero, all its
   predecessors have visibility VISIBLE and non zero alpha, the
   view is not fully covered by predecessors and is within the
   screen.

   The function that computes whether a view is visible for
   accessibility purposes was not taking into account the
   predecessors' alpha.

bug:7454355

Change-Id: I7609f4366da260091d68e5b25832498843fd3d0a