Refactored the FieldsClassification score mechanism.

Before, FillEvent.getFieldsClassification() returned a map of remote ids and
scores. Now, it returns a Map of FieldClassication by AutofillId, which allows
multiple fields and scores for multiple user datas (although the initial
implementation supports only the top match for a field).

This is mostly a refactoring CL, as the implementation is still saving just one
user data entry and one field. But full support is coming next...

Test: atest CtsAutoFillServiceTestCases:FieldsClassificationTest
Test: atest CtsAutoFillServiceTestCases:UserDataTest
Test: atest CtsAutoFillServiceTestCases:FieldsClassificationScorerTest

Bug: 68045531

Change-Id: I08b29f24efbd527216f9bce2343e1bcd4b4554c0