Use push notifications

c2dm is marked as deprecated:

Important: C2DM has been officially deprecated as of June 26, 2012. This means that C2DM has stopped accepting new users and quota requests. No new features will be added to C2DM. However, apps using C2DM will continue to work. Existing C2DM developers are encouraged to migrate to the new version of C2DM, called Google Cloud Messaging for Android (GCM). See the C2DM-to-GCM Migration document for more information. Developers must use GCM for new development.

So you should consider the deletion of the comment about using as3c2dm extension.

More info: https://developers.google.com/android/c2dm/