Minor API changes on Autofill Field Classification:

- Removed Scores class and use float[][] directly.
- Created constant for metadata keys.
- Removed getAlgorithName() from FieldClassification.

Bug: 70291841

Test: m -j update-api
Test: atest CtsAutoFillServiceTestCases

Change-Id: I62abf93c4f6f0804d838c7fb183c0b8bcb88a136