Fix the condition for LLNDK ABI diff

IsLlndk() returns whether the library is an LLNDK stub. The ABI check
should identify LLNDK implementation by isImplementationForLLNDKPublic().

Test: development/vndk/tools/header-checker/utils/create_reference_dumps.py \
      && make
Bug: 227282691
Change-Id: I570871e24d9029992a722b3052f8bb6e7f3c0f52