Fix #3: Test, Build, Sign and push to GH releases

[kumy]

• Fix Attach build to GitHub releases #3
• Relates Ajout d'Olvid sur F-Droid #2

Here is a GitHub workflow that will run on every branch to build and test the APK, and on tag it also create a release, sign and upload the APK.

It needs some secrets to be added to the repo to sign the release (see https://github.com/marketplace/actions/sign-android-release). it also need the GITHUB_TOKEN to have write privileges on the repo.

You can see the result on my repo fork https://github.com/kumy/olvid-android/releases (Using a dumb signing key)

Bests

[kumy]

This workflow still work, due to #13, the "test" step had to be disabled temporarily

https://github.com/kumy/olvid-android/releases

[Neustradamus]

@finiasz: Have you seen this PR?