Ignore not visible views computing click point for accessibility.

In accessibility mode we compute a location to where to send down and up
events to click a view (triggered by a double tap). This solves the problem
of interacting with views that are partially covered by other interactive
views. The logic that computes the click point was not ignoring siblings
that are not visible. As a result a blind user cannot interact with some
views.

bug:17632224

Change-Id: I93e637ee6702e4ba3e79faa736205430e2196d01