This patch adds a GitHub Actions workflow which will automatically run
through all steps to build the Android version of the app. This is an
easy way of catching major build errors in pull requests or pushed
commits.
Additionally, this uploads the Android APK as build artifact. This
allows anyone to easily download any build for a while. That will make
it much easier for non-developers to participate in testing a new
version or a specific test since the no longer have to wait for a new
release.
The build artifacts are a bit tricky to find, but they are easy to point
out if anyone asks.
