Android: Make it possible to override a target's APK directory

Summary:
Some projects such as marble do some operations with the APK directory
before building. This allows us the project to copy the apk directory
over to the build directory and do whatever it pleases.

Test Plan: Built Marble

Reviewers: Frameworks, vkrause

Reviewed By: vkrause

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

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D13403