Function onNotificationOpened

onNotificationOpened(input): EventListenerRemover
Registers a listener that will be triggered when a notification is opened by user.
Parameters
- input: OnPushNotificationMessageHandler
  A callback handler to be invoked with the opened PushNotificationMessage.
Returns EventListenerRemover
- An object with a remove function to remove the listener.
Example
```
// Register a listener
onNotificationOpened(message => {
  doSomething(message);
});
```
Example
```
// Register multiple listeners
onNotificationOpened(message => {
  doSomething(message);
});

onNotificationOpened(message => {
  doSomethingElse(message);
});
```