Cordova file plugin onEvent(such as onload) should be invoked with _realReader as this

When `FileReader` add onEvent listener such as `onload`, it will add the listener to inner `_realReader` , but when it invoke the listener, it use the FileReader as `this`.

1. When add onload listener in application code.

```javascript
var fileReader = new FileReader();
fileReader.onload = function() {};
```

it will call `www/FileReader.js defineEvent`

```javascript
function defineEvent(eventName) {
utils.defineGetterSetter(FileReader.prototype, eventName, function()

{ return this._realReader[eventName] || null; }

, function(value)

{ this._realReader[eventName] = value; }

);
}
```

the listener is added to `this._realReader`.

And when the listener is triggered for example in `readSuccessCallback`
it will invoke the method with `this`.
```javascript
this.onload(new ProgressEvent("load",

{target:this}

));
```

in the normal senario, it will ok, but when it work with some polyfilles such as `zone.js of angular4`, it will fail because `zone.js` have some special handling which require the `this` object be the same when add listener and invoke listener.

https://github.com/angular/zone.js/issues/868