Added a callback for AutoFillService.

So far AutoFillService only received the assist data from framework; in
this CL, it also offers a method where the auto-fill provider can send
the auto-fill data back to framework.

The workflow is:

- AFMSI calls a new AM method (requestAutoFillData(), instead
  of requestAssistContextExtras()).
- The assist receiver is located in the app, not on system service.
- AM uses a new request type (ASSIST_CONTEXT_AUTOFILL) to request the
  assist data to the activity.
- ViewStructure has a new setAutoFillId() method which is used to set an
  unique id for the view.
- View uses the accessibility id to implement the auto-fill id.
- When the activity fullfills the request, it creates an IAutoFillCallback
  remote object - that will be used to set the auto-fill fields - and
  returns it in the assist bundle (using the
  VoiceInteractionSession.KEY_AUTO_FILL_CALLBACK key).
- The app-visible AutoFillService class offers an onFillRequest() method,
  which contains the assist data and a FillCallback used to handle it.

BUG: 31001899
Test: manually built and ran it

Change-Id: I3d208c14e81022dc96dd03f38bbe25a778b24a67