Implement the main switch of notifications on lock screen

Add the ability of controlling whether to show notifs on the lock
screen (secure setting: lock_screen_show_notifications) to the new
lock screen notification settings page main switch. This switch also
listens to the change of lock_screen_show_notifications to update
its status in case of other places change the value.

Bug: 367455695
Flag: com.android.server.notification.notification_lock_screen_settings
Test: atest LockScreenNotificationsPreferenceControllerTest
Change-Id: Id598d508ac383563a0d9413578251eb22ed4ca90