Emit metrics about migration prompts suppressions

The metric is about logging when a certain AutofillAi prompt was not
shown because another AutofillAi prompt was shown, since we only show
one prompt per form submission.

That metric so far supports only save and update prompts, and this CL
adds support to log such information for migration prompts as well.

This required merging the member functions that used to take charge of
(1) save/update (MaybeImportForm()) and (2) migration
(MaybeUpstreamEntityToWallet()).

Bug: 445679087
Change-Id: I2ec6603442584a19413acc0c7c16dec284a5e1ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7054725
Commit-Queue: Jihad Hanna <jihadghanna@google.com>
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1533012}