Gradle & OpenKeychain compatibility

This would have been a maintenance release (because only fixing the OpenKeychain Bug #18), but since this is fixed by the new build system, and this is a major change, it is a major release.

I hope F-Droid is intelligent and detects the new build system without problems, we'll see. Amazon App Store and Play Store releases are already in the pipeline, both signed with the same key as the release apk attached here.