Do not use shared libraries on Android

This is needed for cross-compiling ROS to run on Android devices as we need to use static libraries. Similar to https://github.com/ros/class_loader/pull/20

Is there a specific reason why this lib needs to be hard-coded as shared? I've added the if clause to avoid breaking something else but, as catkin_make's default behavior is to build shared libraries, this seems redundant and maybe we can omit it, or is there something else I'm not considering?

Thanks!