We need to pass around the account ID with onMailboxSelected too.
(It's kinda sad it wouldn't have happened if we had par-account starred
mailbox.)
Also made sure MailboxListItem.mMailboxId now really contains only a mailbox
ID. Before this chage, we stored an account ID to this for an account row
on the combined mailbox.
Bug 4452811
Change-Id: I732fd8eb18f787f4a700a45a40768f96e3bb8751