Fix call to mock.assert_not_called()

There is no assert_not_called() method in mock, as described here:
https://code.google.com/p/mock/issues/detail?id=159
Fix the call to use the mock.called attribute instead.

Change-Id: I4b37764ac65736934fe3448bc50520bcd84ff61d
Merge "Fix call to mock.assert_not_called()"