as far as Mitchel does not want to answer in Premium Forum will cross-post the issue here.
Nevertheless in some post was stated that back button on Android is still supported if use sencha's history facility, I'd argue.I have created a example's kitchensink as a separate project and put it to github. http://github.com/olegtaranenko/kitchensink
you can run this project locally (do not forget setup touch sdk properly) and call it with mobile browser. Android hardware back button does work, perfect! Honestly speaking history support is implemented not ideal (kudos to sencha guys) but at least you can view back is in charge.
Now you can do native packaging, follow with readme instructions.Back button is dead. More precise it close the app immediately.
Looks like Android WebView implementation history is not supported...