Start de-coupling NotifStats from NSSLController.

This is not completely straightforward, hence the various TODOs
scattered throughout the code. This will only be completely de-coupled
once the footer refactor is done, but I'd like to keep the CLs separate
as much as possible.

Several components need to check whether there are any notifications or
not, and they currently do that by looking at
notifStats.numActiveNotifs. Instead, we should have only one source of
truth in the form of the ActiveNotificationsRepository. This CL focuses
on decoupling this dependency.

Bug: 293167744
Test: manually tested that the empty shade and footer behave correctly
in various circumstances; running e2e tests in another CL with the flag
on; NotificationListViewModelTest, ShadeInteractorTest
Flag: ACONFIG com.android.systemui.notifications_footer_view_refactor DEVELOPMENT

Change-Id: Id18abb88db78c736ba788b491d86487068edd2a7