Add create_reference_dumps.py --lib-variant

The new parameter lets the caller finalize LLNDK separately from NDK
and PLATFORM. create_reference_dumps.py filters lsdump_paths.txt by
-libs --lib-variant. It builds and copies only the needed .lsdump.

Test: ./create_reference_dumps.py -libs libnativewindow \
      --lib-variant LLNDK -products aosp_arm64 -release next
Bug: 314010764

Change-Id: I95fb382b90d4ebd12e1a587cd1b6bc2cb1035900