Add an additional mailbox key column to message table

b/11294681
The problem is that when we try to open an attachment for a
message in search results, it fails. The reason is that part of
loading the attachment, we need to open the remote folder the
message is in. For search results, the message's mailboxKey is
the special fake "search_results" folder, which doesn't actually
exist on the server.
For this change, I've added a new column called "mainMailboxKey".
For search results, this column will be populated with the real
mailbox the message is in. It will be blank for other messages.

This is a quick and low risk fix for this bug, but it's kind
of awkward. We would prefer to do one or both of the following
some time after MR1.

1. Make the "search_results" folder be a virtual folder, the same
way that unread, starred, and other virtual folders are. For these,
there is actually no mailbox row in the database, just some
queries that check various flags in the messages and behave
like folders in the UI. The messages actually still reside in the
real folders.
2. Remove the requirement to open the folder at all to load the
attachment.

Change-Id: I825ab846f78bf8b041a5d1d579260dc5d7b4c522
4 files changed