Make native_headers product_available

We're fixing the build system to correctly handle the image mutator and
we must explicitly expose native_headers to product modules because some
product modules use the sdk_version property. (Till now it worked
because product modules incorrectly become the core variant)

Bug: 370351758
Test: build
Change-Id: I0a6efb080be1683d44ee33f2cab7d332b7ab1f9c