Navigator warns about BackAndroid

The "else" code path below is running on iOS, triggering a warning:

``` js
 if (this.parentNavigator) {
 this.parentNavigator.setHandler(this._handleRequest);
 } else {
 // There is no navigator in our props or context, so this is the
 // top-level navigator. We will handle back button presses here
 BackAndroid.addEventListener('hardwareBackPress', this._handleBackPress);
 }
```

cc @ericvicenti 
