MatterControl is open source, meaning the source code is freely available as stipulated by the license under which it is released. The builds for Mac, Windows, Linux, and Android are all compiled by us, but the ones we choose to release for free are the ones for Mac, Windows, and Linux.
The compiled Android build is not released for free, but instead is bundled as part of the MatterControl Touch device we sell. We do not have any plans to release the compiled Android version either for free or as a paid option, but will continue to make it available only on the Touch.
Since the source code is still freely available you technically have the option to compile the code yourself using an Android development environment; we just do not offer any support in doing so.