Rebuild AAR when Java sources change

Summary:
This is a fairly aggressive heuristic, but that's fine as Gradle wont do
much if nothing relevant changed, so it's more important that we don't
miss anything.

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18191