Handle race between creating and assigning RemoteTargetHandles

* Getting a callback from shell to inform when task is in split
and when it's been removed is asynchronous, and not
coordinated with launcher's swipe up gesture.
* There's a chance that user can quickswitch to single app
and start swipe up before shell could notify
LauncherSplitScreenListener, which would cause mismatched state.

Bug: 199658495
Change-Id: I722eeb26d83e99b2a2f77748984d0d7c390b5fec