Android toolchain: allow to specify extra libs manually

Summary: Add new variable ANDROID_EXTRA_LIBS, which can contain list of full paths to libs to include in resulting APK file. This can be used to include plugins that are not directly liked to executable, for example OpenSSL libs for QtNetwork SSL/HTTPS support.

Test Plan: Build project with -DANDROID_EXTRA_LIBS="/path/to/lib.so;/path/to/other_lib.so". Build without it. In both cases verify APK contents.

Reviewers: apol

Reviewed By: apol

Subscribers: kde-buildsystem, kde-frameworks-devel

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D13198