[Android][Extensions] Rename EventHandler's event type and Implement on(at|de)tach function

In http://www.w3.org/TR/2011/WD-html5-20110525/webappapis.html#event-handler-attributes ,
the W3C spec define the Eventhandler and the related type name.
The type name in the ```addEventListener()``` should be ```attach``` instead of ```onattach```.

Add another method to add a listener.
For example:```navigator.system.onattach = function() {...}```