[Dual Shade] Replace the scrim background color to match the spec.

The concept of "notifications scrim" no longer exists in Dual Shade;
instead, we used the color spec for "scrim behind shadePanel" as defined
in the UX spec (#404040 with alpha of 40%).

Fix: 414579329
Test: Manually by opening the shade on both dark and light theme and
 observing that the scrim color behind the shade is correct.
Flag: com.android.systemui.scene_container
Change-Id: Iad367d3d4313836bf2f09704bcec1b3769bd8094