Apptitecture has provided a Mobile App Previewer to its users for the purpose of viewing their Apps as they build them. This is good for Mockup purposes as they can let their Clients view their Apps and even test out the functionalities of the Apps before publishing in the Google Play Store.
Just simply enter your email address and password in the fields provided to gain access to all your Applications for viewing. Your Clients will also be able to download the Previewer.