- fix the AccountManager documentation. http://b/2401790

- only pass the authtoken through from the authenticator to the client
  for getAuthToken() and strip it out from the other calls, like
  addAccount(). http://b/2332762
- beef up the documentation to indicate what calls are allowed to be made
  from the main thread and which are not allowed. http://b/2384961
- wait a bit before retrying syncs that failed because one was already
  in progress. http://b/2414235