Problem with convertFileSrc on android

When doing the following (can be done in the debug browser console attached to and android app):
```
let path = cordova.file.applicationDirectory + "www/" + relativePath;
path = window.Ionic.WebView.convertFileSrc(path);
fetch(path)
```
getting error 404.
I was able to retrieve the file when using `"http://localhost/" + relativePath` which is weird and a terrible workaround...

This is a spin off of #380