Add shell transition handling for staged split-screen.

This makes StageCoordinator implement the TransitionHandler
interface.

In general, this currently expects 'enter' transitions to
contain 2 tasks (one in each split). The current UX is undefined
when one only one of the splits is occupied, so for now it will
throw an exception if that case is hit.

There is a split-screen API called startTasks which takes a list
of tasks (currently only supports 2) and associated options and
creates a transition to both enter split-screen and launch those
tasks into their respective stages.

These are the currently accounted-for entrypoints into
the handler interface:

- Core-initiated (handleRequest)
  - in split: trigger=HOME that is opening -> full dismiss.
  - in split: trigger=task with a stage parent that is last closing in
                    that stage -> dismiss with other stage onTop
  - NOT split: trigger=task with stage parent -> exception
- Shell-initiated
  - NOT split: startTasks -> enter split with 2 tasks
  - in split: snap-to-dismiss -> dismiss with other stage on top

Bug: 182002789
Test: atest SplitTransitionTests
      Or use the experimental pair-launch split-screen and observe
      protologs to see clean transition-infos.
Change-Id: I4f4dd431ad5642cf98b4a01c32eb1d09e5b9a11e