android: build AAR and setup continuous integration

[bassosimone]

We create a new directory called MOBILE. This directory will
contain two subdirs, both gitignored. We will put output
packages inside the dist subdir. We will use the gopath
subdir as the temporary gopath.

The go.sum file changes slightly because we are using
content from the x/mobile repository. We are however not
compiling and building gomobile in a module enabled env
to avoiding tainting go.sum and go.mod too much. We are
in fact using a temporary gopath precisely for building
gomobile outside of any module environment.

Also, there is a build that mimics what I am currently doing
on macOS, to validate we can still build for Android. The build
also produces the .aar and the sources as artefacts, e.g.
see https://github.com/ooni/probe-engine/actions/runs/41802231.

These artefacts are stored at GitHub for 90 days,
which is good enough for the use case of downloading and
testing them. We will need a better and more comprehensive
solution when we build tags.

This is part of measurement-kit/measurement-kit#1913
and specifically it builds a replacement for MK on Android. Of course,
we are not at the point where we can get rid of MK completely yet, but already
we have an AAR that implements some of our experiments.