Allow entering split screen on an app currently open in desktop mode.

Registers a SplitScreenListener to perform necessary windowing changes
when split is requested for an app currently open in desktop mode.

Test: Manual; open a task in desktop, go to overview, perform a split
transition where the second app is the one open in desktop.
Bug: 279586624

Change-Id: Ib00e0fe5d0dd0dfa49b025d78de994fd68af2145