[shade] use black and white instead of onSurface for colors

onSurface doesn't have enough contrast to be visible in all cases.

Test: manual
Test: ShadeHeaderScreenshotTest
Bug: 421970953
Flag: com.android.systemui.shade_header_blur_font_color
Change-Id: I177b174a5e26a2b472884fe7004c9d960889c0e8