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 

The app loading sequence is like this:

1. The Polymer UI loads.
2. The business logic (spark.dart and spark_polymer.dart) loads and initializes `SparkAction`s and other things, which potentially might use elements from the UI via `getUIElement()`.
3. The UI (this class) receives `modelReady()` event and possibly updates the UI based on that.

The problem may arise in step 2 when there is some part of the UI conditional on a flag, like you have here with `<template if="{{apkBuildMode}}"`, and some `SparkDialog` in spark.dart tries to extract elements within that part of the UI. If the flag has the initial value of false, the entire part will not get rendered, and the code in the business logic will fail to find the elements it needs. So: as the comment before all these definitions says, it's best to assign true to all the flags initially (or false if the condition in the HTML is negated), an then let the initialization go through and assign the final value eventually.