[iOS] Fix passkey migration for shadowed credentials

GetPasskeyByCredentialId filters out the results, so we can end
up not finding a credential because of it being shadowed. Instead
of using GetPasskeyByCredentialId to see if a credential already
exists, this CL uses GetAllSyncIds, which returns every sync ids
without filtering them. New passkeys will now properly only be
added if the sync id doesn't already exist in the passkey store.

Bug: 355047898
Change-Id: I172f1981687ee346bf112178f7b9790901d651bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5947843
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1372299}