Add GroupedTaskView for gestures in staged split.

* Currently only works for portrait 50/50 split
* Have gesture swipe animation flow use multiple
TaskViewSimulators, one for each app in split
* Added new APIs in shell to query for actively running
tasks to determine if we're in split screen
* Lots of UI polish needed during gesture
* Launching into staged split after live tile ends
not implemented yet.

Bug: 181704764
Change-Id: Ib90e99e1e10b19121e8709385e1334b9380d6502