Skip animation background if there is a wallpaper

This improves the animation for the case where the wallpaper target
is CHANGE-ing (remaining visible) as part of an OPEN/CLOSE transition.

In this case the wallpaper will be a better animation background than
a solid color, unless any of the participants specifically asked for
a solid color.

An example of a case this improves is BlurTests. Every case launches a
translucent dialog on top of an opaque activity and then finishes
the opaque activity first. This generates 1x CLOSE without a wallpaper
flag and 1x CHANGE with a wallpaper flag, which wrongly gets
interpreted as not showing wallpaper.

Flag: EXEMPT bugfix
Test: atest ShellTransitionTests
Test: atest DefaultTransitionHandlerTest
Test: atest BlurTests
Bug: 227736816
Change-Id: I7296118f59aa592bc48df9fc8c037ad5dd5e6ed2