[CodeHealth] Remove uses of ListValue::Append in /chrome/browser/ui/webui/settings/chromeos.

This CL removes uses of the overload that accepts a
std::unique_ptr<base::Value>. It attempts to modernize code where
feasible and uses base::Value::FromUniquePtrValue where it's not.

This CL was uploaded by git cl split.

R=zentaro@chromium.org

Bug: 1187097
Change-Id: I5ec8f6108d8d896d6c656cc82d55f293f32402de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3626094


Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Auto-Submit: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999899}