The problem was that CombinedMailboxesLoader used the cursor returned by
super.loadInBackground() (which contains accounts), to build a matrix
cursor (which contains special mailboxes and accounts and will be returned),
and *it closed the first cursor* after building the matrix cursor.
However, because this first cursor is the one that CursorLoader sets an
observer, it shouldn't be closed until the returned matix cursor closes.
In other words the two cursors should have the same lifecycle.
Fixed it by using ClosingMatrixCursor that used by AccountsLoader, which
is doing a similar thing, but properly.
Bug 3387730
Change-Id: I554ade001dc25afa869eefb4dcf9887495e6753e
d0ee1e404d