Remove BackAndroid.

This will throw an error in `react-native@0.58+`.

Author: hyochan

index.js

@@ -10,7 +10,6 @@ var {
   TouchableWithoutFeedback,
   Dimensions,
   Easing,
-  BackAndroid,
   BackHandler,
   Platform,
   Modal,
@@ -19,7 +18,7 @@ var {
 
 var createReactClass = require('create-react-class');
 
-var BackButton = BackHandler || BackAndroid;
+var BackButton = BackHandler;
 
 var screen = Dimensions.get('window');