Fold SeenNotificationsProvider into new NotificationListInteractor.

This is just renaming/restructuring things a little bit for the
FooterView refactor.

Bug: 293167744
Test: tested manually that "Unlock to see older notifications" shows up
correctly + existing tests (no behavior change)

Change-Id: I421ad493150e32ec8271484a5d41afac969e7bcd
8 files changed