Use the LayoutManager as the source of truth for visible positions

During animations, the view group child may not correspond to the
visible views that appear at the top.

The previous logic is kept in the case that the layout manager returns
null for any reason, which can happen before layout has occurred for the
position.

Fix: 189588014
Test: verified locally
Change-Id: Ie8b5dcef50287e9e90a21f86e30a1ebcbbcba30f