Android: Don't hardcode a random version of the Android SDK

Summary: Just default to whichever is installed

Test Plan: Built kalgebra

Reviewers: Frameworks, vkrause

Reviewed By: vkrause

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

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D14544