Fix issues with mailbox additions

* Make mailbox changes synchronous (the reason for it being async
  hasn't be relevant for a while); there is a potentially BAD race
  condition here.
* Fix test for user mailbox (was completely wrong, which caused new
  generic mailboxes not to be recognized

Bug: 6430910
Change-Id: I7f9ccb143b9232a2694ee105bde21180bf2bf33e
1 file changed