Support Gradle reproducible builds

[MariusVolkhart]

In Gradle 3.4, support was added for reproducible builds. https://docs.gradle.org/3.4/userguide/working_with_files.html#sec:reproducible_archives

The ProGuardTask is not a AbstractArchiveTask, and so using the recommended way of achieving reproducible builds is not possible.

This feature request is to enable reproducible builds when using ProGuard. Ideally, it would be possible to do so using the recommended Gradle mechanism, but if a custom configuration is required, that is acceptable as well.