[customize chrome] Fix the focus ring's color in the theme color picker

This CL fixes crbug.com/414764619 by removing the enclosing div
container of the custom color picker, i.e., by following the solution
mentioned in the bug.

The reported issue occurred because of the following:

- In the original code, the enclosing div element has its tabindex attribute set, and thus it is considered focused when the focus is moved.
- cr-theme-color's boundary color (box-shade) is determined by
--cr-focus-outline-color[1], while the div does not have this css
applied

The fix adopted here is to slot the custom picker icon into
ThemeColorElement, using cr-theme-color directly at the top-level with
all the attributes set to it. This causes the following behavior
changes:

- the cr-theme-color element becomes focusable
- when focused, the element is styled with the css that is consistent with the other grid elements.

The resulting border color is confirmed manually[2].

[1]:
https://source.chromium.org/chromium/chromium/src/+/main:ui/webui/resources/cr_components/theme_color_picker/theme_color.css;l=21;drc=d4c10b208905b9e6e715fa20e853700295375821
[2]:
https://screencast.googleplex.com/cast/NDY1Nzc5NzcwMTU2NjQ2NHwwYTUyOTI5ZC1mNg

Fixed: b:414764619
Change-Id: Ib645a3610a11edcc391fe8ded471a4326280ac8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7009000
Reviewed-by: Riley Tatum <rtatum@google.com>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Sho Mizoe <shomizoe@google.com>
Cr-Commit-Position: refs/heads/main@{#1527659}