Break circular dependency in NotificationEntryManager

The NEM has a dependency on the ranker that it really shouldn't have
(because the ranker has a dependency on things that might want to have a
dependency on the NEM). This makes it a lazy dependency for now -- this
should be a field that gets set on the NEM eventually.

Bug: 186481467
Test: atest
Change-Id: Ic5192c787ad367fadfc1e43eb529f7663ab81d77