Refactored field detection mechanism to support multiple fields.

Now instead of using FillResponse.setFieldDetection() with an object that
contains both the autofill ids an the user data, service must:
- Set global mapping through AutofillService.setUserData()
- Pass just the autofill ids to FillResponse.setFieldClassificationIds()

This CL is purely a refactoring CL - although the new API supports multiple
values and multiple fields, the implementation is still hardcode for one
value and one field (as before).

Test: atest CtsAutoFillServiceTestCases:FieldsClassificationTest
Test: atest CtsAutoFillServiceTestCases:UserDataTest
Test: atest CtsAutoFillServiceTestCases:FillResponseTest

Bug: 68045531

Change-Id: I8d846d817dfcde3a4db7abff798bb2250e1c4e7b