[Divider] Fix flicker when dragging to dismiss secondary container.

Previously, the surface decor was removed too early in
expandTaskFragment() when there are containers to finish. It should instead be done in TYPE_TASK_FRAGMENT_VANISHED after the task fragment is gone.

Before: https://screencast.googleplex.com/cast/NTgyOTIyNTkyNjg4NTM3NnxmY2YxNDA3MC0yYQ
After: https://screencast.googleplex.com/cast/NTU4MTIyMTQwMzgyMDAzMnw4ZjUwYzkyYi1lOA
Bug: 343509560
Flag: com.android.window.flags.activity_embedding_interactive_divider_flag
Test: atest DividerPresenterTest SplitControllerTest
Change-Id: If507a09791b5b29724625c6b2bc24c492f4b3495