Remove NotImplementedErrors from "virtual" methods

Raising a NotImplementedError in a method of a base class
makes it impossible to use super() with it and do multiple
inheritance properly. Those methods should instead simply
return some default "empty" value, so that the methods in
the subclasses can still call them, if that's where they
fall in the MRO.

Change-Id: I581391e87dc58f0f76adea3dbf90cef173d3daef
Closes-bug: 1278825
Merge "Remove NotImplementedErrors from "virtual" methods"