stripped down Android build process for gitlab-ci and Vagrant

This makes the Android build process be based on the reproducible Debian packages as much as possible, including for the Android SDK and golang. It also adds a Vagrantfile which will create a VM based on the android: job in .gitlab-ci.yml. That makes it easy to use the process as defined in .gitlab-ci.yml on a development or release build machine, for example. This also is almost generating reproducible builds, but it seems that gomobile has a bug that prevents it: https://github.com/golang/go/issues/40254