Set clock and date color in ShadeHeader

Clock and date color should be:

Collapsed:
Light theme: black
Dark theme: white

Expanded:
colorScheme.onSurface

The underlying scrim will provide enough contrast for the time/date to
always be visible against the user's wallpaper.

This is currently broken when Quick Settings is fully expanded in light
theme, as the underlying scrim is not properly visible. See: b/399112904

Bug: 433932732
Test: clock/date text color is set correctly in quick settings
Flag: com.android.systemui.scene_container
Change-Id: Ie289d14ac38d6eb984c06c3d41926c81ac493d5e