[Notif redesign] Use onSurface for hybrid notif text

The color for all other text in notifications should have been updated
in I09007d3da23e4fb3f56e94b18218aea5554bd3e2 since onSurfaceVariant no
longer meets a11y requirements, but this one was declared separately.

Bug: 409496809
Test: manual
Flag: android.app.notifications_redesign_fonts
Change-Id: I93fb44af2e7617fea04c43d89350268a76fde034