Make NotificationSettingsRepository hold individual settings.

Settings can change independently, so it's not very practical to have
them all in the same object.

Also moved NotificationSettingsRepositoryTest to SystemUITests (for now;
see b/315806189)

Bug: 293167744
Test: NotificationSettingsRepositoryTest + verified that wallpaper
picker notif toggle works correctly
Flag: NONE

Change-Id: I5a927edac3ee5d4cb35dcc12e944d40e69c03743