[iOS]Disambiguate Local and Identity reauth coordinators.

Having both ReauthCoordinator and ReauthenticationCoordinator was
confusing. Especially given that they were doing very different kind
of authentication.

Hopefully, the new comment will help disambiguify, and redirect the
reader to the correct coordinator if they got it wrong first. And the
new name, stating whether the authentication is related to the
SystemIdentity or to the iphone validation.

Bypass-Check-License: Moved files
Bug: None
Change-Id: I70ba088c8754b23b977d22d22ab35a073a7a3b75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6942992
Auto-Submit: Arthur Milchior <arthurmilchior@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Ernesto Izquierdo Clua <eic@google.com>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1516523}