Split activity organizer in WM Extensions

Reference implementation for the client-side organizer
for side-by-side split activity UI.

Bug: 190433398
Bug: 190432728
Bug: 190433500
Bug: 190433972
Bug: 191163577
Bug: 191163638
Test: Manual, using the sample in WM Jetpack
Change-Id: I27335d47f4f544e2b9f63c5daaac9bd1c272a89f