Send demo country and device locale in SetUpDemoAccount request

This change addresses an issue where Google web apps within a signed-in
demo session do not respect the system language set. Currently, demo
accounts are created with English defaults because the client does not
send locale and country information to the backend.

This CL updates `DemoLoginController::SendSetupDemoAccountRequest` to
include the device's locale and demo country in the JSON payload.

This allows the server to provision the demo account with the correct
country and language preferences, ensuring a localized experience in web
apps.

Bug: 448204936
Change-Id: I0c03bc0051c36c37107ae088597623c0f704dde5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7002019
Reviewed-by: Xiqi Ruan <xiqiruan@chromium.org>
Commit-Queue: Haifan Wang <wanghaifan@google.com>
Cr-Commit-Position: refs/heads/main@{#1525925}