[Android] Fail more gracefully when building with API < 23

Summary: Building the Android backend with SDK < 23 is successful but fails at runtime because the jar is not found. Instead of just not including the jar when < 23 don't include the backend at all.

Test Plan: Itinerary now doesn't crash at startup

Reviewers: vkrause, apol

Reviewed By: vkrause, apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17973