[flexiglass] Make shade header colors match the UX spec.

On Single/Split shade, the shade header foreground color should be
white on Dark theme, and black on Light theme.

Fix: 438407920
Test: Manually tested by opening notifications shade and quick settings
 shade in Single and Dual shade modes, on both dark and light themes.
Flag: com.android.systemui.scene_container
Change-Id: Id9e4443f87052302fc3045b61141f921e33e32e9