Octave for Android
The Android app Octave in Google play is built and maintained by Corbin Champion and not part of the GNU Octave project.
The source and some build instructions for the installer can be found here
This link might have some other, perhaps now outdated, build instructions:
You are offered a choice of downloading the app, for which a donation is requested, or you can download the full source and build your own version for free. This will require the Octave sources, Corbin's GitHub repository, and the Android SDK.
This implementation has been done in close cooperation with the Octave developers and makes use of the current Octave source essentially without any change. Thus, it is fully compatible with the versions of Octave on other platforms.