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 

I don't think this APK specific dialog belongs in the `ProjectLocationManager` class. Perhaps add a new method to the utils.dart `Notifier` class:

    Future<chrome.ChromeFileEntry> chooseFileEntry()

This could then have a concrete implementation in spark.dart.