Define install location for JAR/AAR files for Android

Summary:
This follows where Qt installs those, and thus androiddeployqt looks for
them. Currently this is hardcoded in knotifications for example, seems
cleaner to have this here.

Reviewers: Build System, Android, apol

Reviewed By: Android, apol

Subscribers: apol, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D23131