Merge "Samples for L Notification and unit tests. Basic requirements follow the PRD(https://docs.google.com/a/google.com/document/d/1bL_2PFXEAdyK7Ih2p6K_PD4OQMzFFNWHr4jtQ3U5WUk/edit#heading=h.uumdtaoatylu), slightly changed the phrasing from Security -> Visibility since the current implementation of Notification is using Visibility." into lmp-preview-dev