[ios] Fix DemographicsClient to work with multi-profile

The code using DemographicsClient only access the SyncService and
the Preferences when there is a single profile on disk so enforce
this in the DemographicsClient.

Remove the usage of GetLastUsedBrowserStateDeprecatedDoNotUse to
access the profile (and instead access it via the list of all
loaded profiles).

Fixed: 355629111
Change-Id: Iba0a9bdd4ede2bfd59175dca411e309435e48ac7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5821932
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1348176}