Decouple Dataset ids from SaveInfo.

The FillResponse was automatically adding the AutofillId of all Datasets to
the SaveInfo object, but that would cause problems when the AutofillService
is not able to save some data (for example, if it comes from a read-only
backend).

Bug: 36076444
Test: OptionalSaveActivityTest pass
Test: m update-api
Change-Id: I1d5faaddf29e1be0f357438c8485e07caf975293