MailboxFinder should ignore callback for non-target account

There were two cases where MailboxFinder responded to updateMailboxListCallback
when it shoulnd't.
- Callbacks for non-target accounts
- Callbacks arrived after the operation is finished

Make sure these callbacks are properly ignored.
Also, make sure startLookup() can't be called more than once.

Change-Id: I823c11ab5f96df4eb84594c08d3325d12319f708
5 files changed