Historical notification access API.

Similar to getActiveNotifications(),
getHistoricalNotifications() returns a list of all
notifications that have been posted, in
reverse-chronological order. It currently includes duplicate
entries for notifications that have been updated (so it
really is tracking every notification that has been posted
to the system).

Change-Id: Icce8d6f96bbe76710c989fd0068ff971c6498605