Handle Zen Mode computations correctly for updated notifications.

Updates to intercepted notifications come in through addNotificatiom
instead of updateNotification, because the intercepted notifications
are not in mNotificationData.

If an update causes a notification to surface, we need to notice and
remove it from the synthetic notification.  However, addNotification
is called from within InterceptedNotifications inside loops over
mIntercepted, and we cannot remove items from mIntercepted while
looping, so we split addNotification into two parts, one part is only
used by InterceptedNotifications to unconditionally surface previously
intercepted notifications.

Bug: 15389069
Change-Id: I7b0952a337f15d4009e3e61360344012345eac95