Settings: Fork settings-captions UI between Browser and CrOS Settings.

This is the last piece of UI that is hackily shared between CrOS and
Browser settings, and unfortunately is not possible to share it via
the "limited sharing" mechanism. Forking seems the best path forward to
fully decouple the two UIs and unblock remaining tasks.

As part of the forking
 1) removing all code guarded by "not is_chromeos" guards from the
    CrOS Settings fork.
 2) limiting the forked files to "not is_chromeos" on the Browser
    Settings build (they were incorrectly included in other builds)

Bug: 1431477
Change-Id: Ic5b8a426ee57cb911b8858ea6a9e369c0dcec037
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4544887
Reviewed-by: Evan Liu <evliu@google.com>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146123}