Autofill Field Classification improvements.

* Changed the remoteId -> userValue API to userValue -> categoryId so the
  category could map to multiple values (for example,
  "email" -> "email1", "email2")
* Added method and settings for maximum number of category ids.
* Tuned the default value of some settings.

Bug: 70407264

Test: atest CtsAutoFillServiceTestCases:UserDataTest \
            CtsAutoFillServiceTestCases:FieldsClassificationTest \
	    SettingsBackupTest

Change-Id: I27f348c500077937c0f4bf65db6a899fa3c41cf6