Android: Allow passing a relative path as the apk dir

Summary:
This way the project can generate automatically its own manifest file.
GCompris is using it to set the version.

Test Plan: Built GCompris without a warning saying the apkdir isn't found

Reviewers: Frameworks, GCompris, jjazeix

Reviewed By: GCompris, jjazeix

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

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D15643