Add back dismissing parallax and dimming value to stage split

Add back dismissing parallax and dimming value to stage split as what we
did for legacy split screen.

Fix: 195699800
Test: atest WMShellUnitTests
Test: observe it applies dismissing parallax and dimssing value
      properly.
Change-Id: I7aa414cd79fab1c625f95c17fea33598d84fa55e