healthd_internals: Extract color from data series

To support data series from different sources in one chart, we should
decouple the color from the DataSeries class.

Bug: none
Test: autoninja -C out/Default chrome browser_tests
Test: ./out/Default/browser_tests -ozone-platform=headless \
    --gtest_filter="*HealthdInternalsBrowserTest*"

Change-Id: I53a4b563b8e9c79a46b2680a7ccddba1716ff815
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6037465
Reviewed-by: Denny Huang <dennyh@google.com>
Commit-Queue: Byron Lee <byronlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1388095}