Add quick and dirty async AssistStructure building.

New APIs on ViewAssistStructure all the app to request to
build a sub-tree asynchronously and indicate when it is done
with that.  The overall AssistStructure is now only flattened
and transfered on-demand, when the app receiving it requests
its data -- and at that point we can wait for any asynchronous
building to complete.

New AsyncStructure view is a very simple example of using this
to asynchronously build a child view.

Change-Id: I14f9199bee64915ad3dc80b2190916ec874308af