'Notification Listener' doesn't work.
I fixed the example which is described in developer guide to work fine.
These are the main points of this correction.
* specify namespace for NotificationFilter.
* set 2 blank lines between class definitions according to E302 of pep8.
* change argument of 'pool' to keyward arguemnt, because 4th argument of
'get_notification_listener' requires the name of message executor.
Change-Id: I75800309f2294fe9f17ea2a2b1638de57ef2922e
Closes-Bug: #1618394