FIX: Call all android back key handlers before invoking default

Summary:
Fixes issue #9736
Closes https://github.com/facebook/react-native/pull/9757

Differential Revision: D4164796

Pulled By: hramos

fbshipit-source-id: c5d966a9bbaa91f9929ea20a775f96c15aa9f482

Libraries/Utilities/BackAndroid.android.js

@@ -41,6 +41,8 @@ RCTDeviceEventEmitter.addListener(DEVICE_BACK_EVENT, function() {
 /**
  * Detect hardware back button presses, and programmatically invoke the default back button
  * functionality to exit the app if there are no listeners or if none of the listeners return true.
+ * The event subscriptions are called in reverse order (i.e. last registered subscription first),
+ * and if one subscription returns true then subscriptions registered earlier will not be called.
  *
  * Example:
  *