Detect duplicate submissions more accurately

This CL:
- Changes AutofillAgent::submitted_forms_ from a set<FormRendererId> to
  a map<FormRendererId, set<SubmissionSource>> to be able to get more
  information about previously detected submissions

- Adds logic in AutofillAgent::FireHostSubmitEvents that more accurately
  filters duplicate submissions along with documentation.

Bug: 40281981
Change-Id: I5ea4f505312783d58db4ced55603ee4bf0eafa1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5812089
Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Jihad Hanna <jihadghanna@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1348336}