Settings.Global.LOCK_SCREEN_SHOW_NOTIFICATIONS.

  $ adb shell settings put global lock_screen_show_notifications 1

Previously a per-user secure setting
(Settings.Secure.LOCK_SCREEN_ALLOW_NOTIFICATIONS), but this
doesn't really make a lot of sense since this behavior
pertains to areas of the system (keyguard, status bar) that
exist outside of any one user.

This setting now effectively enables notifications on the
secure lockscreen for all users.

Change-Id: I9c5179498dbbba9343184efdedb1587ea862cf96