Fix BackAndroid subscriptions calls

Summary:
`BackAndroid` JS event subscriptions should be called in reverse order
(the subscription from the latest `addEventLister` should run first).

Also if listener returns true, don't call other listeners.

**Motivation**: We use `BackAndroid` listeners to prevent closing screens with user's input.
When we have two screens in stack (each screen with listener, which show alerts),
we want to show alert only from the last screen listener, not from all.
Closes https://github.com/facebook/react-native/pull/8929

Differential Revision: D3598978

Pulled By: dmmiller

fbshipit-source-id: a7b0762b36a60755a844e90fffd58887f89c9ffb

Libraries/Utilities/BackAndroid.android.js

@@ -25,11 +25,14 @@ var _backPressSubscriptions = new Set();
 RCTDeviceEventEmitter.addListener(DEVICE_BACK_EVENT, function() {
   var backPressSubscriptions = new Set(_backPressSubscriptions);
   var invokeDefault = true;
-  backPressSubscriptions.forEach((subscription) => {
-    if (subscription()) {
+  var subscriptions = [...backPressSubscriptions].reverse();
+  for (var i = 0; i < subscriptions.length; ++i) {
+    if (subscriptions[i]()) {
       invokeDefault = false;
-    }
-  });
+      break;
+    };
+  }
+
   if (invokeDefault) {
     BackAndroid.exitApp();
   }