Store demo mode country preferences in uppercase.

Manually tested that email is case insensitive: registered a test device with admin-CA@cros-demo-mode.com is assigned OU ca.

Bug: b:180732658
Change-Id: I4e3c77218b67106df997fb7f76bab9cc093c8e4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3251967


Reviewed-by: Yves Arrouye <drcrash@chromium.org>
Commit-Queue: Yves Arrouye <drcrash@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958205}