Do not create the animation background layer...

...if there is only one change in the transition.
The Launcher's root task does not participate in the second transition,
which is triggered by finishing the ProxyActivityStarter. This should
be fine as the Launcher had already participated in the first close
transition. However, because the root task isn't part of the second
transition, that transition contains only a single "close task" change.
This causes the animation-background layer to incorrectly occlude the
Launcher layer.
There is no need to create the background layer because no other
changes can interact with the single change.

Bug: 438014794
Flag: com.android.window.flags.polish_close_wallpaper_includes_open_change
Test: Close edit widget activity ProxyActivityStarter, verify no
animation-background layer occludes Launcher.

Change-Id: I7d67d32da7f253bea2f1d3c0d16ab10d4703681b