Add support for dragging third app on top of 2 app split

* Lots of UI polish TODO
* Dropping doesn't work, this is just to add plumbing to
support UI for views

Bug: 349828130
Flag: com.android.wm.shell.enable_flexible_split
Test: Things work fine when flag is off

Change-Id: Icdbbef013f054996d9c26dfaeaa22f534210e44a