Changes on Assist structure to handle virtual child for autofill:

- Removed overloaded newChild() methods.
- Added a setAutofillId(parent, virtualId).
- Changed view so AutofillId is set on all relevant methods.

Test: VirtualContainerActivityTest (with new tests) pass
Bug: 36056207

Change-Id: Ia11344f95c2756e83307c54052878e9dbe471873