Bug 1060541 - [Lockscreen] Adding 1 additional notification to lockscreen with new music player

The notification builder should keep as simple as possible. So I would avoid to manipulate container here. If this is necessary, fire an event to LockScreenNotifications to handle it is better. Since 'builder' should only build and fire event when it's necessary, such like the callback it hook on the certain elements. To manage container is the duty of manager (LockScreenNotifications).