Threads moved to a watched page don't trigger notif.

The page moving is a bit confusing in lqt. When moving a page
the Message Controller is now invoked two times, once for the target
page (this change) and once for the old page.

If a user is watching both pages he will get notified twice.

Bug: 30698
Change-Id: Id9fb014ef20b42a3aae608fc31a12036888859c3
Merge "Threads moved to a watched page don't trigger notif."