[ReactNative] Replace Backstack with BackAndroid

Libraries/Utilities/BackAndroid.ios.js

@@ -0,0 +1,21 @@
+/**
+ * iOS stub for BackAndroid.android.js
+ *
+ * @providesModule BackAndroid
+ */
+
+'use strict';
+
+var warning = require('warning');
+
+function platformWarn() {
+  warning(false, 'BackAndroid is not supported on this platform.');
+}
+
+var BackAndroid = {
+  exitApp: platformWarn,
+  addEventListener: platformWarn,
+  removeEventListener: platformWarn,
+};
+
+module.exports = BackAndroid;