SystemUI support for notification visibility.

In this implementation, DISABLE_NOTIFICATION_TICKER (which was never
really used on its own and can be safely subsumed by
DISABLE_NOTIFICATION_ICONS) is now DISABLE_PRIVATE_NOTIFICATIONS;
when this SystemUI bit is set by the keyguard, SystemUI knows to switch
its presentation into "public" mode, in which
VISIBILITY_PRIVATE notifications are replaced with their
publicVersion's contentView (or a placeholder view,
synthesized by SystemUI, that leaks no additional
information about the notification). VISIBILITY_SECRET
notifications are suppressed altogether in this mode.

This behavior is enabled but not activated by default. To
turn it on, run:

  $ adb shell settings put secure lock_screen_allow_notifications 1

and restart SystemUI.

Change-Id: Id660bef7737580e16a83f60567c22b53ee81c602