Copy visibility docs from support version to base Notification.
Copy visibility docs from support version to base Notification.

(Noticed the difference in a javadoc diff between Notification and
NotificationCompat)

Bug: 17424399

Change-Id: I639a46c429ffebf8ca47118b2ea80f40ccdc1286
xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base