There's no need to create a URI just to rip in appart again. Additionally, to
support additional changes (i.e. to use Mailbox instead of Folder in the
MessageController), we need to store the actual Account.
NOTE -- This change only affects IMAP and POP3. SMTP will come in a follow-on CL
Change-Id: I400036a17271c99272fd9c603547dcd713b50b9d