Moves snapViewIfNeeded() call to listener on NSSL.

This change removes the logic from NotificationEntryManager which snaps
an updated notification's view back into place and replaces it with a
NotificationEntryListener defined by the NotificationStackScrollLayout.
This allows us to remove a method from the NotificationListContainer
interface and gets us halfway to breaking the dependency from
NotificationEntryManager on NotificationListContainer.

Test: atest SystemUITests, manual
Change-Id: I744ece3fa660454d9725aa553ffebcfe8846a303