Fix allowed STL types for system libraries.

Static libraries using the system STL have type
native:ndk:system:none, not native:ndk:system:shared, since no
actually linking takes place. These are still allowed to be linked to
platform modules.

Test: make checkbuild
Bug: None
Change-Id: Ib999bb0f422ec3938daa740fca5681e153d6525a