Notif pipeline: call on UpdateFinished() callback after adding a child.

This used to be called in
NotificationViewHierarchyManager#updateNotificationViews, but in the new
pipeline world we no longer use that and instead rely on
RootNodeController now. Give the callback a heads up after
adding a child.

Bug: None
Test: Build
Change-Id: I4a55fdc59d405bc552f47d14d6ccafa0fb9e042f