Pass const FormData& in AM::Observer::OnFormSubmitted.

This allows observers to analyze the submitted values of the form -
the FormGlobalId alone would not allow that because the FormStructure
that can be looked up using the id is not updated when the observers
are called.

Bug: 343124027
Change-Id: Ieb2d58cc53bfc3589caec288cda1b1ba39f536c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5578079
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: Jan Keitel <jkeitel@google.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Peter Conn <peconn@chromium.org>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1307353}