Prevent setting launcher layer has opaque when there is content behind

- This can prevent the layered content behind to not be visible (and
  since launcher draws a cutout and the wallpaper isn't shown, may
  result in a black rect.  We only specialize for this case while
  Launcher is in Overview.
- Also don't need to defer updating drawsBelowRecents, this can result
  in the state not being reflected since it runs after the last update
  of the transform params

Bug: 205789573
Test: Swipe up from app and ensure behind layers are visible (also
      ensure this doesn't affect blur during the swipe or optimizations
      after you leave overview)
Change-Id: I07689b3d9b65708797576e5fbefe12fb1f544119