Let SplitScreenController handle the split tasks.

On split to desktop transition, we remove the leftover split screen
task. This shouldn't be done for 2 reasons:

1) StageCoordinater already reparents the task on exit, so we shouldn't
   need to close it.
2) removeTask() removes the task from recents as well. Expected behaviour is that the leftover split task is moved to recents.

Test: Move from split to desktop using the handle/menu/keyboard
shortcut. Leftover split app is still in recents.
Bug: 326043729

Change-Id: Ie0414f7672f244260c975120ce27f473d0676527