Don't install system image copies of libraries in unbundled apex builds

Apex builds are setting LOCAL_REQUIRED_MODULES to include external
dependencies into the system image.  This make sense for device builds,
but just causes extra unnecessary builds for unbundled apex builds
that are not building device images.  Skip the dependency during
unbundled builds.

Test: aosp-master-art mainline_modules_x86_64-userdebug builds
Change-Id: I363995b7309cc3c90b0a584d051050f105b5b915