Create NotifGroupManagerHelper interface

This fixes group expansion for the new notif pipeline.

Creates a new interface, NotifGroupManagerHelper, which helps
controllers determine group information (ie: summary, parent, children)
given a notification entry. This is necessary for parts of the
notification pipeline that are shared between the old and new pipelines
that require querying group information (ie: HighPriorityProvider,
PeopleNotificationIdentifier).

Many parts of the old pipeline directly inject
NotificationGroupManagerLegacy. This indicates that when migrated to the new pipeline,
we can delete the old group manager's functionality since the new
pipeline handles the logic elsewhere (most likely in ShadeListBuilder)
For example, the new pipeline no longer needs to
track isolated entries nor suppressed summaries.

Test: test SystemUITests
Change-Id: Id716f881aa7122c08be6dada8fb43058594d89f1