Move various pipeline classes into packages

Keep the base statusbar.notification.collection package *only* for
classes that need to access package-private members of
NotificationEntry/GroupEntry/etc.

At some point, we should move NotifEntry et al into their own package
like "core" but that will involve a massive scary CL.

Test: atest
Change-Id: I10808f8ce6bd4ae4ce8c1ac194bde6b8a6fea758