Ensure notifs are filtered when device is locked

...even when the device is not presently on the lock screen (ex:
occluded by Camera)

Fixes: 234327150
Test: manual
  1. Post a notification w/ Notify.apk
  2. Enable setting to hide notifications on the lock screen.
  3. Lock the device
  4. Launch camera from the lockscreen
  5. Open the notification shade.
  Observe: No notifications are present in the shade.
Change-Id: I33a40e6e9cec3caf8e485ea942d70458c9af887c