Add Fenix (Firefox preview)

Please note that according to Sören Hentzschel (comment in German), the APK will only be uploaded/published to the GitHub release page once rollout on Google PlayStore has reached 100%. Between official release and 100% rollout, the GitHub API will point to the new release, but the APK isn't uploaded yet. In this time frame, downloading the APK from GitHub will fail. This is also the case as of writing this pull request on 2020-05-10 20:30 UTC.

/close #11 (closed)