Experiment: hide unused channels in settings

behind a 3-dot menu. "unused" in this case means a channel that is not blocked but also hasn't sent a notification in the last 2 weeks

Test: manual
Flag: com.android.server.notification.notification_hide_unused_channels
Bug: 322536537
Change-Id: I99f5a61411c9b3a567fd7517f0bf7ea33bb7637c