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Make the flingFinishedCallback be executed
Exit split if the side stage no longer has children. We should invoke flingFinishedCallback even if there is no need to run animation when fling divider to dismiss. Bug: 244540465 Test: Drag Chrome to create multi-instance in split screen repeatly Change-Id: I101044515c366287e37951a32a1b57618f1c8f3e