demo_mode: Send demo account requests with device serial number

Use serial number as device unique identifier for demo account requests.
Server side will use it for account allocation.

Bug: 372762477
Change-Id: Ib9a8b32fd0ff89c3794f077433922a8ca9481d7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6034923
Reviewed-by: Haifan Wang <wanghaifan@google.com>
Commit-Queue: Xiqi Ruan <xiqiruan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1386501}