SystemUI: Follow light/dark theme in SplitShade Header

* Google's default implementation is dark, which means
  it doesn't need to follow the light/dark color change,
  but we broke it, so add it to complete the SplitShade
  Header color change

Co-authored-by: Col_or <col_or@qq.com>
Change-Id: I5464039885197eeb43bd31b822bfcba7a1b08776
Signed-off-by: Saalim Quadri <danascape@gmail.com>