First of all, it seems you are working with the old Smart Extension SDK - you should upgrade to the Sony Add-on SDK v2 to get the newest APIs for the newest Sony Mobile accessories, and the updated emulator.
Regarding your question(s) - the way the notification API basically works, you add notifications to the Smart Connect notification database via a Content Provider. Once the notification has been added, all connected Sony Smart accessories will display the notification. To add this functionality to your application, you can go about it in two ways:
- Add the extension functionality to your already existing app. By doing this, you will not need to send the String (that you mention) between app processes.
- Create a separate notification extension which takes the information from your app.
For both implementations, you can use the SampleNotificationExtension project that is included in the SDK to start your work from.
Hope this helps, cheers!