Android: Better support for add-on library projects

When including add-on library projects (such as google-play-services_lib), this can be done quite easily by copying the project into your android source directory and adding a reference to it in project.properties.

However, one extra step is required, which makes the source dependent on local paths: You have to run android update on the library project. This step can easily be done by androiddeployqt to make it simpler for users to include third-party libraries in their builds.