Suggest plus addresses on Username / Single username fields

The PlusAddressSuggestionsOnUsernameFields feature introduced here aims
to replace AutofillGivePrecedenceToEmailOverUsername.

The logic will be almost identical meaning that if a field has a
server prediction USERNAME or SINGLE_USERNAME and the heuristics
detected it as an EMAIL_ADDRESS field. The plus address should be
suggested.

The difference between this and the previous feature comes from the
fact that the previous implementation suggested email addresses coming
from autofill alongside the plus address suggestion whenever such
scenario occurred.
That caused KeyAutofill metrics to get diluted as many of the password
manager forms were now included in them too. This feature prevents that
behaviour and instead offers plus address and the autocomplete
suggestions.

Bug: 360791229
Change-Id: If53209dabe6c3a384c3bd1c5b56834eddc8423d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6198510
Reviewed-by: Norge Vizcay <vizcay@google.com>
Commit-Queue: Karol Sygiet <sygiet@google.com>
Cr-Commit-Position: refs/heads/main@{#1414191}