Partially fix #2455341 (Exception adding event w/o DTSTART)

* Apparently, the Exchange server can send event changes that have
  incomplete data.  This is not documented, and the result is very
  bad in that an exception is thrown by CP2, and the sync (and
  future syncs) is prevented from finishing
* Wrote some defensive code to detect bad events and simply not
  add them
* Open a new bug to track trying to determine why the Exchange
  server is sending this kind of data.

Bug: 2455341
Change-Id: Ibcfd412382164351c96f368dc043f38c7b481154
1 file changed