Avoid wrapping the display change leash to Keyguard unlock animation

..., in case the animating root task being occluded by this change.

Also skip other non-tasks and non-wallpaper changes as they don't
need to animate for keyguard unlock animation.

See: http://recall/-/bN53iXCwu0DIehsmJlTyWk/eefM8O857S8JUBJmQCieKK

Bug: 283963801
Test: manual by issue steps:
  1) Launch apps to enter split-screen mode
  2) Turn off/on the screen with power key to show the lockscreen
  3) Rotate the device in landscape
  4) Swipe up to dismiss the lockscreen
  5) Expect the split-tasks animates fade-in without seeing a weird
     cross-fade out animation on top of the split-tasks
Change-Id: I261e081b1cb0bd17d0523e2c80368a669a6246d6