Extract the mailbox lookup logic into an independent class

This new class MailboxFinder is responsible for looking for a mailbox
by an account id and a mailbox type.

If a mailbox is not found on the first try, it'll tell Controller
to refresh the mailbox list, and try again later.

This will be used by MessageListXL.

Change-Id: I4adc3db025fb271c254aa2b58b3b753281dc7398
4 files changed