Add LLNDK stubs and headers to VNDK snapshot

LLNDK stubs and headers are required to build the vendor modules
against VNDK snapshot libraries. Add the LLNDK stubs and headers to
the VNDK snapshot prebuilt files.
The stub libraries will be included in shared/llndk-stub directory.

Bug: 181815415
Test: development/vndk/snapshot/build.sh --build-artifacts
Change-Id: If518f3e91080e69fa1da94af0aa27320d4e71a08