Add meetingInfo column to Message; use for meeting invites

* Added a meetingInfo column to the Message database
* When a meeting invite is received, the start time is stored here
  in ms from start of epoch.  Note that this field is defined to be
  a String, for extensibility
* Update ProviderTests

Change-Id: If44892d27ccc5ebdc1f8667befafb8b8a27a2cf4
3 files changed