Summary:
`BackAndroid.addEventListener()` returns a subscription object with a `remove()` function in android. Before this fix, the iOS equivalent doesn't return anything, which means, if there's a component doing something like this, it would redbox:
```
componentWillMount() {
this._subscription = BackAndroid.addEventListener('hardwareBackPress', () => {...});
}
componentWillUnmount() {
this._subscription.remove(); // --> redbox in iOS before this fix
}
```
Differential Revision: D3790480
fbshipit-source-id: 1e607171bf2892a6b64977c4fd052c5df0bc4a0d
150fe7cb9c