Make time and date up date font colors update on theme change

Previously, the colors did not update because the background was
a fixed black. Now that background blur is enabled, the font color
needs to update with the theme/mode like the QS tile fonts.

Bug: 405266952
Flag: com.android.systemui.shade_header_font_update
Test: added new tests; manually verified with QS tile
Change-Id: Ibf1b7cf808161cad142c8d76fd4ad42d17a24b50