Skip collecting requesting-invisible wallpaper if no target

The transition of wallpaper target may have completed and then
the wallpaper target relayout to invisible which triggers refresh
new wallpaper target. If there is no new target, the wallpaper
will set to invisible. So it should not belong to any transition.
Otherwise the next transition may include wallpaper unexpectedly.

Fix: 334578612
Test: atest ActivityTransitionTests# \
      testAnimationBackgroundColorIsUsedDuringActivityTransition
Change-Id: I6db11773a1fe4aeb05b748b3c76424994befe0ce