Send email related to event exceptions

* We weren't sending out appropriate emails for exceptions and other
  event updates
* Write exception specific ics file code in CalendarUtilities (in
  the existing ics file creator)
* Send appropriate Update: subject for updated events/exceptions
* Compose simple message text consisting of:
  When: <time>
  Where: <location>
* Prepend message text for exceptions to indicate that the message
  relates to a particular instance of the event:
  This event has been canceled for: <date>
  The details of this event have been changed for: <date>
* New strings were added in CL#44141
* Updated CalendarUtilities tests

Bug: 2501270
Change-Id: I920de8120bc56d5bd565cbde26ff4807be41579f
3 files changed