Added new API to set field classification algorithms per category.

Originally one field classification algorithm was used to classify every
field. The change allows different category of fields to be classified
by different field classification algorithms.

Change-Id: I27205a4096774d6e0c0d56da5e0fd38dda995d8f
Fixes: 118681526
Test: atest CtsAutoFillServiceTestCases
Test: atest android.autofillservice.cts.FieldsClassificationTest