migrate: Migrate static functions in AshColorProvider

1. Created the `ColorUtil` class.
2. Migrate the color related static functions GetBackgroundThemedColor
   and GetColorProviderSourceForWindow from `StyleUtil` to `ColorUtil`.
3. Migrate the color related static functions GetDisabledColor and
   GetSecondToneColor from `AshColorProvider` to `ColorUtil`.

Fixed: 1350550
Test: Manual
Change-Id: I70b67049faea59e2f11a66e58ed974ce4c55c939
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3877266
Reviewed-by: Gordon Seto <gordonseto@google.com>
Commit-Queue: Connie Xu <conniekxu@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: Tim Sergeant <tsergeant@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1046167}