Fix drag-to-dismiss split screen crash with shell transition

There is no need to update split layout after the fling to dismiss split
animation finished. Remove it to prevent conflicting with split
dismissing shell transition.

Fix: 208954095
Bug: 206872147
Test: atest WMShellUnitTests
Test: manual check dismissing split with shell transition won't crash
Change-Id: Ie5faf521fa47e8e534382b6feb63fc27c14627b3