Do not check the ABI stability of LL-NDK-Private

This commit skips the ABI checks on LL-NDK-Private because
LL-NDK-Private libs are only used by other VNDK-core or VNDK-SP libs on
the system partition, are NOT used by vendor modules, and do not
constitute a system-vendor interface.

Bug: 122938657
Test: development/vndk/tools/header-checker/utils/create_reference_dumps.py
Change-Id: Ia2af4250ef1443f8ea3ed5ab111668462f120979