[Autofill] Extract datalist for AskForValuesToFill()

Before this CL, AutofillAgent::QueryAutofillSuggestions()
retrieved the datalist suggestions for the queried field separately,
except if Config::extract_all_datalists is true.

After this CL, it retrieves the datalist suggestions for all fields.
The expected overhead of this change is minimal because if there is no
datalist attribute, GetDataListSuggestions() boils down to constructing
and iterating over an empty vector.
The change simplifies QueryAutofillSuggestions() a little and avoids a
const_cast.

Bug: 40232021
Change-Id: Iecabee2d6f198798132aa273c60a08189b5fed03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5612630


Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Auto-Submit: Christoph Schwering <schwering@google.com>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1312658}