Changes to N Preview samples page.

- Removed JobScheduler, for now. Since the major update is coming
  as a followup update, let's save announcing this until its ready,
  for PR purposes.
- Swapped "Notifications" sample for "Active Notifications" and
  "Messaging Service", to reflect actual sample list.

Bug: 27556402
Change-Id: I0b62854bf5e0fc5180c2048861b20631e41c1718