Handle viewing of attachments that are, themselves, emails

* Add intent filter for application/eml and message/rfc822 mime types,
  launching MessageView with a Uri
* Modify loadMessageTask to handle the Uri by parsing the attachment's
  input stream with Pop3Message.parse(), and then creating an
  EmailProvider message in a special Mailbox created to hold
  "attachment" messages
* Delete all "attachment" messages after the parent message is closed
* Add unit tests

Change-Id: I20276ee006b9f05b889f3c808d3dc407cde26d49
4 files changed