Apk Build UI 

Regarding  #3351
![screenshot from 2014-09-09 16 54 46 v01](https://cloud.githubusercontent.com/assets/8182792/4210875/08b1dca0-387d-11e4-8463-3119a6d60989.png)

review @ussuri @devoncarew 

No need to change this here, but just as an aside: `_` is often used to indicate "I know there is a parameter I'm supposed to take, but I'm not going to use its value". So: `getElement('#choosePrivateKey').onClick.listen((_) {...});`.