[Android]: Store message when application is on background

See [AGPUSH-1043](https://issues.jboss.org/browse/AGPUSH-1043)

## Step to reproduce

* Send a message when application is in background
* Dismiss the notification on notification bar
* Send another message when application is in background
* Click on notification on notification bar 
* The application need show 2 messages
should we perhaps rename the `sendNotification()` to something else? Name might imply it actually sends a message (from the device to somewhere else)