Focus search in AbsListView returns invisible views.

1. The focus search algorithm considers only visible
   views, specifically when adding focusables. Since
   AbsListView implements a custom focus search strategy
   for better user experience it also has to return
   only views that are visible.

bug:6543878

Change-Id: I6207c29402e4ac6190c1e65d042e04449aa75ef8