Add funtional animation for SplitSelect from Grid.

Long ways to go to final UI - TODO(b/186800707)
Add comments to grid layout code,
no functional changes at all.

Bug: 181705607
Test: Enter split screen from overview task menu
in grid and carousel. Able to select all items
and enter split.

Change-Id: Ib62f1b286acf0781ec47862fa31b670e6ff1892a