Avoid duplicate upper/lower case locales in localeToDisplayName.

I confirmed in the debugger that we previously were storing both upper
and lower case locales in localeToDisplayName (e.g. `en-us` and
`en-US`). This change converts it all to lower case for consistency.

Bug: 352583619
Change-Id: I9b46cba870686c7e1816c827410846a6e1495b1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5721773
Reviewed-by: Kristi Saney <kristislee@google.com>
Commit-Queue: Eitan Goldberger <eitang@google.com>
Cr-Commit-Position: refs/heads/main@{#1330185}