Move channel listing into a pref controller

And a hidden preference category. This makes
hiding/showing the list a lot cleaner and also allows more
of the code to be tested.

Also delete some unused code that no longer complied after
this refactor.

Fixes: 133443871
Test: atest
Change-Id: I4a5fe0e075019bae2df44a0a9dcec26a40ee6d12

src/com/android/settings/notification/ChannelNotificationSettings.java

@@ -16,6 +16,8 @@
 
 package com.android.settings.notification;
 
+import static com.android.settings.notification.ChannelListPreferenceController.ARG_FROM_SETTINGS;
+
 import android.app.settings.SettingsEnums;
 import android.content.Context;
 import android.content.Intent;