All (targetting Android): Add a new setup method to initialize the Android version without contacting the GCM.

Also, add the method to iOS for symmetry, although it is a no-op there.

This new setup method is specially useful on coldstart or background situations,
since the notification can be processed even if the network is unreachable.

Background:

In our use case (I think it is quite general), calling GCM also requires us to provide the tokens to the server so it can deliver the notifications later on. This network use is most often not required (and potentially slow, depending on signal quality), since the tokens will remain valid, so we decided to not "register" every time the app is started. 

On Android, skipping the call to register stops the notifications from being processed since the reference to the WebView and ECB are not configured.