Provided an AutofillCallback API.

Some custom views - like WebView - might have their own auto-complete mechanism,
so we need to provide a way for them to know when the auto-fill UI is shown or
hidden.

Fixes: 35948429
Test: CtsAutoFillServiceTestCases (with new tests) pass
Test: m update-api
Test: manual verification

Change-Id: I5682a3b9645d5d077a4a2446e79256d6f77b4b5a