Avoid continue reparent divider leash after dismiss

Actually splitscreen is still not stable for entering or closing but handle the recents request with onRecentsInSplitAnimation
but may divider leash already been released and warning in onRecentsInSplitAnimationStart, need add this check in onRecentsInSplitAnimationFinish too.

Bug: 406889299
Test: repeatly splitscreen entering/closing transition and enter recents
Flag: EXEMPT bugfix

Change-Id: I6ef8d11478d69ef0675f9b64064192f520396b7f