[signin] Make locale optional in AccountInfo

Do not require `locale` to be filled in `AccountInfo::IsValid()` as this
field might not be populated by the UserInfo.

Bug: 330384267
Change-Id: I6acdcaa95d79ff9dd9bfd0fdb1c1ec93925bd0fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404793


Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Auto-Submit: Alex Ilin <alexilin@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280196}