Added DiscourseLogger, which stores last N (currently 64) lines of IMAP
commands sent to the server and responses received from the server.
We dump it to logcat when the IMAP parser crashes, that is, a) getting a
RuntimeException in ImapFolder.fetch() or b) getting a Runtime/IOException
in ImapResponseParser.
Bug 2480227
Change-Id: I6b5a728a7df106627ec29bb3c7c04a97a99b444b