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