LLNDK can re-export headers

Added export_llndk_headers properties to llndk_library module. And a new
module type llndk_headers is added. This is to enable an LLNDK library
to reexport other LLNDK headers.

Bug: 65395259
Test: do the following
// frameworks/native/libs/arect/Android.bp
llndk_headers {
    name: "libarect_vendor_headers",
    export_include_dirs: ["include"],
}

// frameworks/native/libs/nativewindow/Android.bp
llndk_library {
    name: "libnativewindow",
    ....
    export_llndk_headers: ["libarect_vendor_headers"],
}

check that
-Iframeworks/native/libs/arect/include is in LOCAL_EXPORT_CFLAGS of
libnativewindow.vendor in out/soong/Android-<product>.mk

Change-Id: If1650414b2967f2042f4ebe2b593ed3f3ea45d3a