Add a new color token and update another

Add the cros.sys.system-on-base-opaque color.  Update the
system-on-base token.

Change-Id: Ie9d015533b8938d93191fbb1282a8fc3e9032bb2
Bug: b:277140063, b:277140832
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4426131
Commit-Queue: Sean Kau <skau@chromium.org>
Reviewed-by: Zain Afzal <zafzal@google.com>
Cr-Commit-Position: refs/heads/main@{#1130625}