Fixed race condition when calling onCompleteCallback.

[danilo04]

Fixed race condition when calling onCompleteCallback. The execution of the callback should not be tied to a timeout, should be executed after the email was sent or an error is catched. It is mentioned in javier-moreno's message in Design of Library issue.

The fix changes the behavior of onCompleteCallback to just set the field successCallback. In the block that sends the email, a finally block is added to call the method callOnCompleteCallback. This method does the same as the previous implementation of onCompleteCallback but gets the Handler of the UI Thread. By executing the callbacks on the UI thread, we avoid any errors if the callbacks (onSuccess or onFail) update UI elements.

[nedimf]

Thank you