Get rid of VirtualViewDelegate.

VirtualViewDelegate was used by views providing virtual children to:
- auto-fill the chidren
- notify system about children changes

Since the notification logic moved to the AutoFillManager class, it's
kind of an overkill to provide a delegate just for autoFill(); instead,
it's simpler to add a new autoFillVirtual(virtualId, value) method on
View.

BUG: 31001899

Test: manual verification
Test: CtsAutoFillServiceTestCases passes

Change-Id: Ia7670aa665918496a009017a11c3de72c4ed3fa2