Bug 1062807 - Use the new shared helper for loading JSON files in the wallpaper app

Converted the Pick.js class to LazyLoader.

https://bugzilla.mozilla.org/show_bug.cgi?id=1062807
nit: Don't split declaration and assignment of the variables, you could have kept the existing code as is and just moved the function below the existing declarations. E.g.:

```javascript
var self = this;
// Get #-moz-samplesize media fragment to downsample while decoding
// so that we can display smaller images without using lot of memory
// See Bug 1011460
var sampleSize = Downsample.sizeNoMoreThan(this.thumbnailScale);

LazyLoader.getJSON(...);
// etc...
```