Add back removal notice of BackAndroid

Summary:
This was deleted as part of 774cd73663

I forgot that we normally keep the getter at the bottom of the module with an invariant so that people get clearer messages when they upgrade.

Reviewed By: yungsters

Differential Revision: D10475667

fbshipit-source-id: fcc527cd8117a506096a910a602272e092b8672d

Libraries/react-native/react-native-implementation.js

@@ -330,6 +330,14 @@ module.exports = {
   },
 
   // Deprecated
+  get BackAndroid() {
+    invariant(
+      false,
+      'BackAndroid is deprecated and has been removed from this package. ' +
+        'Use BackHandler instead',
+    );
+  },
+
   get Navigator() {
     invariant(
       false,