Alternative workaround for using private attribute

- SysUI studio has trouble referencing this private attribute
  directly, so move the resolving of the color to the code
  instead

Bug: 152086714
Test: Studio builds, numpad colors still reflect the
      colorSurface color when changing dark/light &
      wallpapers
Change-Id: I9a96c89010c41eaf6d14d5e1cfe86777cc30c8be