Token-Changed Event Not Firing on Android

### Problem description
When the Firebase token is updated, the on-change event isn't firing on Android.

### Environment
- Tabris.js version: 3.0.0-beta2
- Devices:
- iPhone 7 with iOS 11.1.1
- HTC Desire 530 with Android 6.0.1

### Code snippet
```javascript
/* see existing token */
console.warn("Existing token: " + firebase.Messaging.token);

/* ask for permissions */
if (device.platform == 'iOS') firebase.Messaging.requestPermissions();

/* add on-change event for token */
firebase.Messaging.on('tokenChanged', ({token}) => {
console.warn("Got updated firebase token: " + token);
});
```

#### Output on iOS:
```html

> Existing token: undefined

> Got updated firebase token: ...
```
#### Output on Android:
```html

> Existing token: null

> Existing token: ...
```

### Workaround
By setting an interval that repeatedly checks `firebase.Messaging.token`, you can essentially figure out when the token is updated. This creates needless overhead, though.