FileWriter is not implementing EventTarget

# Bug Report

## Problem

### What is expected to happen?

According to the provided [TypeScript definitions](https://github.com/apache/cordova-plugin-file/blob/master/types/index.d.ts): `FileWriter extends FileSaver` and `FileSaver extends EventTarget`. [`EventTarget` interface](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget) contains `addEventListener` an `removeEventListener` functions.

### What does actually happen?

Functions `addEventListener` and `removeEventListener` are not defined in the `FileWriter` instances.

### Command or Code

```
fileEntry.createWriter(function (fileWriter) {
// It will throw an error about undefined addEventListener on the next line.
fileWriter.addEventListener('writeend', function () {
console.log("Successful file write...");
});
});
```

### Environment, Platform, Device

Samsung tablet with Android 5.

### Version information

```
"cordova-android": "7.0.0",
"cordova-plugin-file": "^6.0.1",
"ionic": "^4.2.1",
```

## Checklist

- [x] I searched for existing GitHub issues
- [ ] I updated all Cordova tooling to most recent version
- [x] I included all the necessary information above

Start at the FileWriter implementation reached through fileEntry.createWriter and compare it with the inheritance and methods declared in types/index.d.ts. Reproduce the provided addEventListener example on the reported Android setup, then verify that FileWriter instances expose both event-listener functions and support the writeend callback.

Written by the indexing model from the issue text.