Polish close translucent task animation when combines with other changes.

When closing a translucent activity, it typically triggers a single
"close" transition. However, if that activity also launches another
activity, the original task beneath the translucent activity will also
participate in the transition, affecting the z-order of the transiton.
- Do not adjust focus task while finish activity, which can affects the
order before next change become visible.
- Keep the wallpaper always at the bottom in Shell.
- Choose cross-fade animation when a close wallpaper transition includes
an open target.

Flag: com.android.window.flags.polish_close_wallpaper_includes_open_change
Bug: 421436197
Test: 1. Launch a translucent activity.
2. Finish the translucent activity, and then launch a new task from it.
3. Verify the translucent activity remain on top, and play cross-fade
animation between open-close targets.

Change-Id: I1f6d52af00c50dc8c83b3eaceabab8950e15e3a0