[Divider] Fix flicker when finishing dragging

This change ensures that the decor surface change happens after the
transition is finished. Before this change, the decor surface was moved
before the transition was sent to Shell, which caused a flicker when
finishing dragging because the decor surface was unboosted but the
TaskFragments bounds were not updated and the snapshots were still
displayed.

Bug: 327067596
Test: atest TransitionTests

Change-Id: I3286023767992ec762bc9459d3f6b2a746209930