Add fastlane metadata generation for Android builds
This is currently done on the signing machines as part of the F-Droid nightly pipeline, but should rather happen as part of the build process in the future.
Compared to the binary factory script this has a few extensions already:
- Besides recovering information from APKs we can now consume appdata files directly, or scan the entire source dir.
- Screenshots from appdata files are downloaded.
- The 'x-test' language is ignored.
- Donation and translation information are added.
- Add links to the source code repository, if we can determine that.
The result is put into a single archive per APK, so we can easily transfer that to the signing machine via Jenkins alongside the APK.