There is no need of any icon launcher, layout, etc.
Yes, there is.
I just need a Broadcast Listener to listen to my incoming messages and perform some sound and vibration activity.
First, on Android 3.1, any such BroadcastReceiver
that you register will be blocked from receiving broadcasts until the user launches one of your activities. Hence, you need an activity, and for the user to be able to start it -- otherwise, your app will never run.
Second, unless you are a malware author, you need an UI for:
The user to configure the behavior of your background processing
Showing online help and support contact information
Displaying your license agreement, copyright notice, attributions, etc.
So, yes, you need an "icon launcher, layout, etc.".