Initial commit of new split screen work.

TODO:
* Extract out common elements in TaskShortcutFactory.MultiWindowFactory
for commonalities between new and old way of
invoking split screen
* Integrate with WM APIs (b/182002789)
* Write tests for SplitSelectStateController

Bug: 181704764

Change-Id: Ice35adb4ea82897f5e2433dc9b93a549f3d511b5