Fix reconnection failure upon network disconnect(Issue#584) (#612)

* Fix reconnection failure upon network disconnect

1. Set default value for clean session mqtt option
2. Update mqttWebSocketURL to use HTTP Port

* Update AwsIotEndpointUtility.java

Remove unused variables

* Use default value for clean session

Use the default value for clean session option without setting it explicitly

* Update AWSIotMqttManager.java

* set default cleanSession to true