Include shell+stage-split in legacy recents animations

This just builds the infrastructure and then, just as
an example/proof-of-concept, uses the infra to show/hide
the divider-bar when it makes sense.

Bug: 192292305
Test: enable SPLIT_SELECT, enter split, use recents and observe
      the divider bar appropriately showing/hiding
Change-Id: Iea0704b3f7821c3056a1675f11e671e81305831f