Revamp view manager logic to support any kind of view

Everything in the notif shade must now be represented by a controler
(specifically a NodeController). ExpandableNotificationRows are
NodeControllers, the NSSL is a NodeController (well, we wrap it in
something that is one), and, in the future, headers will have their own
controllers.

Split NotifViewManager into two pieces:
- ShadeViewManager, which consumes the shade list generated by the
  ShadeListBuilder and generates a "node list" -- instead of
  NotificationEntries, a tree of controllers (and their associated
  views). Plus, in the future, any header views.
- ShadeViewDiffer, which consumes a node list and applies any changes
  that need to be made to the view tree.

Test: atest
Change-Id: I016ff8c454ef7d18d4ac927c45ad132a3e5d4b13