Implement the show sensitive content toggle

Implement the show sensitive content only when unlocked toggle on the
notifications on lock screen page. The toggle will become invisible
when the main toggle is unset or when there's no screen locks.

Bug: 367455695
Flag: com.android.server.notification.notification_lock_screen_settings
Test: adb shell settings get secure \
 lock_screen_allow_private_notifications <1|0>
Change-Id: I174478540c4cc2e2df7047049671266041dd41cf