need to improve security document in rcljava android examples.

@jacobperron   hi jacob, i take the issue here:https://github.com/ros2-java/ros2_java/issues/197,    my real mind is that when we need add security model through rcljava android examples, like set some security args, we miss a document to discribe how people can do it. 

the example miss this case, do we need to improve it ? 