Add divider bg placeholder to app pair icon launch

* Extract out both the top level split root leash
as well as the left and right split root leashes
* Set windowCorner radius on the left/right roots

Test: No more flicker near divider when launching app
pair (see comment for video in CL)
Flag: EXEMPT bugfix
Bug: 349387413

Change-Id: I6fce290d1e7766e55c2f11cc8c365f0c29087c0f