Fixing black background when going back from assistant.

- When returning to a non-wallpaper task from the assistant, the
  opaque animation background for the task-close animation was
  drawing behind the assistant stack and occluding the fullscreen
  task. The original change in ag/99642 no longer applies since
  our task switcher has changed and we can remove this opaque
  background.

Bug: 63072954
Test: Open fullscreen task, open assistant, hit back. Also ensure
      that launching and closing new tasks don't result in additional
      tasks being visible.

Change-Id: I3347a552b15d3cd5ed51f283b99e4caa5eb298a0