Replace calls to Mock.assert_not_called

Mock.assert_not_called() isn't a valid mock method. This replaces
those calls with self.assertFalse(<mocked-obj>.called).

It also fixes one of the tests that was incorrectly (trying to) assert
using the wrong mock object.

Change-Id: Ieecf23de335b15df5811d57ecf9ddd3a30919fa2
Closes-Bug: #1370583