Initial implementation of the Auto-Fill Framework classes.

This CL provides the initial, skeleton implementation of the Auto-Fill
Framework classes:

- Defines the system service and app-based
  AIDL (IAutoFillManagerService.aidl and IAutoFillService.aidl respectively).
- Defines the 'adb shell cmd' interface.
- Defines the permission required to access the service.
- Registers the service on SystemServer.
- Adds the code to bind the app-specified service to system_server.
- Defines the service class (AutoFillService) required by providers.
- Implements the initial startSession() method.

This is still a very early, "work-in-progress" change:
- It has many TODOs.
- It does not have unit or CTS tests yet.
- It does not provide a callback method to auto-fill the fields.
- In fact, it has a lot of TODOs.

Despite these adversities, it can be tested by following the steps
below:

1.Create an app with a service extending AutoFillService

2.Implement the onNewSession() method

3.In the manifest:
 - Listen to android.service.autofill.AutoFillService intents.
 - Require the android.permission.BIND_AUTO_FILL permission.

4.Explicitly set the app as an autofill-service by running:
  adb shell settings put secure auto_fill_service MY_APP/.MY_SERVICE

5.Start a session against the top activity:
  adb shell cmd autofill start session

BUG: 31001899
Test: manually built and ran it

Change-Id: I00f4822159b31ddddba8f513e57c4474bc74eb89