AutofillFieldClassificationService: Mark constructor as @SystemApi

The class is abstract and marked as a SystemApi, but there's real
way to use it while compiling against the system_sdk unless the
constructor is also public.

Test: make update-api
Test: make
Bug: 138589409
Change-Id: Id1fe158ec55b299325bfbc16ab1ea33cd53fc123
3 files changed