Fix cancellation messages for deleted attendee

* We weren't sending a proper ics file for the deleted attendee, and
  this caused Exchange to send a message to the wrong people (the
  referenced bug)
* Split out code that adds attendees to outgoing mail
* Changed the optional last argument to createMessageForX to be a specified
  attendee, i.e. the only addressee to be used for the message

Bug: 2548465
Change-Id: I629fcfaffe621408ea460d42c9c7c283929f7e79
2 files changed