RFC: Error on jars

Patch series which promotes the warning on jars and aars present before building to errors.

Patches 1 (warn on aar) and 2 (remove gradle-wrapper.jar) should be safe to apply right now. Patch 3 is the tricky one and will probably break a few builds. (i.e Nextcloud & owncloud still have one jar).

There will probably a few library candidates that are not available on bintray that are included in a few apps. Let's test locally with this patch applied for a while and see what comes up.