Add the new settings page for lock screen notifs

Add a new empty settings page, which is flag-guarded. When the flag
is disabled, show the original Notifications on lock screen
tri-state switch, otherwise show the new page.

Bug: 367455695
Flag: com.android.server.notification.notification_lock_screen_settings
Test: atest LockScreenNotificationsPreferenceControllerTest

Change-Id: I8a81ad651626f39380767b2c9cfa2f1c19d0e7c5