migration: Set themed color as `user_color` of ColorProvider::Key

This cl sets the themed background color as the `user_color` of
ColorProvider::Key. In the follow up, it will be used to initialize
the colors of Shield and Base layers.

Bug: 1340362
Change-Id: I27bfff02c2b75d3158084211c4176ed50f3a28a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3736546
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1022745}