Interface for notification collection being used

Add an interface for the active collection that is managing the list
of notifications that will be shown on screen.

This saves us from having to do a flag check in each class that can
potentially be used from both NotificationEntryManager and
NotifPipeline and makes it easy to swap them all out for
NotifPipeline once everything is eventually moved over.

We switch NotifRemoteViewCacheImpl to use this interface so that
NotificationContentInflater can now work in the new pipeline without
entry mismatching throwing things off.

Test: smoke test with Notify apk
Test: atest SystemUITests
Change-Id: I8e6a054562b9e7d0ddb385e8d35d81c3b1698833