Implement the Hide seen notifications toggle

Implement the Hide seen notifications toggle on the notifications on
lock screen page. The toggle will become invisible
when the main toggle is unset or when the lock screen notification
minimalism feature flag is disabled.

Bug: 367455695
Flag: com.android.server.notification.notification_lock_screen_settings
Test: adb shell settings get secure \
 lock_screen_show_notifications <1|0>
Change-Id: I6f94be3295dfbaa978d145d113e03cacc1a232fc