Make NotificationStatsLogger and its ViewModel non-optional

After inlining the NotificationsLiveDataStoreRefactor flag, the new
NotificationStatsLogger should be always created. It was just made
optional to make sure that we're not constructing it with the flag being
off.

Bug: 424001722
Test: smoke test the new NotificationLogger
Test: check lights out mode
Flag: EXEMPT mechanical refactor
Change-Id: Ibad2e4a249ac9b233231699a1d965394bb4528e0