[flexiglass] Changes shade and QS background color.

Fix: 322484532
Test: manually. First changed the color to red to make sure the correct
background area is modified. Then tested the change on both light and
dark theme.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I0ddce6bd9fce2b058652afec0c3871885650a94d