[ios] Cleanup SessionRestorationObserver API

Convert all public observer to implement the new API and to use the
helper function to register the observer. Remove the old methods and
mark the new one as pure virtual.

This is part of a multi-CLs change:
  -  https://crrev.com/c/4905372
  -  https://crrev.com/i/6506796
  => https://crrev.com/c/4902850

Bug: 1383087
Change-Id: Idfd428a56f869431d7a6c49a269a8f273ff219b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4902850
Reviewed-by: Federica Germinario <fedegermi@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1203947}