Avoid register but StageCoordinator not initialled

While systemui restart, the main thread may register the
splitscreen animation listener before wmshll.main complete
the initial of handler StageCoordinator, just keep same with
api registerSplitScreenListener.

Bug: 431929394
Flag: EXEMPT bugfix
Test: try repeatly kill systemui with worse performance

Change-Id: I3bcdf4cc0dbfe87614ad3fafb54432b5fd8e028d