Allow building the app easily without having a signage key

Currently Gradle is set up that it expects that there is a file, that contains the signature key for the app. While this can be any key, it must be there. Else you cannot build the app. This is a problem to generate a clean way to add the sailtrainer apps to F-Droid and have a reproducable build there.

Update the build.gradle so that all targets that don't need to sign anything work even when there is no keystore available.