Limit findWallpaperTarget's NotificationShade case

- There is a special case added for ignoring the notification shade
  while the keyguard is going away. This was actually always ignoring
  the notification shade if there's another (activity) window on
  screen which is problematic if that activity is going away and leaving
  just the notification shade on screen.

  + Check for keyguardGoingAway before going into this special case.
    This stops some flickers during the activity/keyguard unocclude
    animation.

Test: atest CtsWallpaperTestCases
Bug: 315698413
Bug: 312649396
Change-Id: I7187e2c392a8d64a1260fc9b5c00225ea79edca7