Drag-to-desktop: cancel gesture goes to fullscreen if from split.

If user is starting a drag from splitscreen and drag to the upper area
to cancel enter-desktop, this currently goes back to splitscreen.
Instead we will start a new transition at the end of the cancel
transition that expands the split task back into fullscreen.

Add also a TODO to actually have split -> fullscreen as one seamless
animation in the future.

Recall http://recall/-/hkw45bbkR06eCMwasLDKpi/dldqIGkWm6nibTBavQqFPC
Bug: 396438812
Test: Manual. Drag splitscreen to center upper, see it goes to
fullscreen.
Test: atest DragToDesktopTransitionHandlerTest
Flag: EXEMPT bugfix

Change-Id: If51609785f6c5b94a03b7fae70a84f5e8e42cbe1