Enable split screen button in handle menu.

Allows apps to begin split select from caption handle menu. This is
compatible with both apps in fullscreen and freeform.

Still to implement/known issues:
- Different flow for desktop to split select when > 1 app is open on desktop.
- Transition currently breaks when app chosen in split select is also open in desktop.

Video: http://recall/-/gjymLwjdDT07aWqaK6101a/gU56zTDcWov6ukbKuH8tFx
Bug: 286550932
Bug: 279586624
Test: Manual
Change-Id: I32cbc44e4cd115e08a351943b1f39dbc116c55c1