You may have noticed that this reposity does not have a typical license. This means that the work here is, by default, under exclusive copyright of the repository owner, and no one but the copyright holder may legally use, copy, distribute, or modify the contents of this repository without express permission. More info here. However, as per section D5 of the GitHub terms of service, you may still view and fork this repository, as well as "use, display, and perform" its content. I believe this means you can legally clone the repository and build the Android APK from source if you wish (for yourself, not for redistribution). Note that the Github ToS does not include the right to redistribute the code.
When making a pull request to this repository, you will be asked to agree to a relatively short Contributor License Agreement (CLA). Agreeing to the CLA simply grants any rights you hold over your contribution to the project owner. This is purely so that I maintain ownership of this project, and so that I can ensure that I remain the sole distributor of the Android Sia app, through the Google Play Store or otherwise.
I have absolutely no malicious intent with this lack-of-licensing and CLA. My goal in making the app's GitHub repository public is for the community to have the ability to audit the code for trustworthiness, and make contributions if they wish. To my understanding, this is the best way to do that while still maintaining ownership over the project and its distribution. App cloning/copying/reuploading is quite rampant in the Android world, and I wish to prevent that from befalling this app.