Filter out non-incoming messages using a subquery.
(because the message table doen't have the mailbox type.)
I was initially thinking of adding a new content URL for
the message table joined with the mailbox table, but it turned out
to be a bit of pain, so ended up using a subquery.
(one of the problems was that both tables had the "_id" field.)
Bug 3177220
Change-Id: I276efb70db1589835f3ddb8c7da4773e72d8691b
6f3d167cfa