Summary:
If we are using the same handler for different events, e.g. both `notification` and `localNotification` use `_onNotification()` handler, the former listener stored in `_notifHandlers` would be overridden by the latter so it's impossible to remove the `notification` listener when we call `removeEventListener`.
This PR stores the listeners by using `pushNotificationEventName` (notification, localNotification, register or registrationError) as the key.
Use the same handler for `notification` and `localNotification`, both listeners will be removed when calling `removeEventListener`.
Closes https://github.com/facebook/react-native/pull/10776
Differential Revision: D4168722
Pulled By: hramos
fbshipit-source-id: d68581428d2acde314f7b5333feafe1ec7807159
e51cb349ce