As far as I know, web apps don't have direct access to or control over the Android back button. I think the solution is to add history events, because in browsers the Android back button defaults to navigating back in history, but this web app already seems to be doing that.
it's actually not on my nexus 4 and 7. which is why I made the comment.
the web apps I make with angularjs do.
also this isn't about direct access to hardware buttons. hitting the system back button on android devices while a web view is in focus is the same as :
window.location.back()
I recommend you look at how angular handles this.