Return nullptr from FindNamespaceByClassLoader for NB-enabled apps.

This function is always called by the frameworks in order to
initialize vulkan layer path. For NB apps returning nullptr
disables vulkan layer library lookup as expected.

Bug: http://b/32542970
Test: mm
Change-Id: I426b2b9f5ea7227b00f063ed6040f37bb2b4f21f