RocksDBKeyedStateBackend snapshot uses incorrect key serializer if reconfigure happens during restore

As titled, in current RocksDBKeyedStateBackend we use keySerializer rather than keySerializerProvider.currentSchemaSerializer(), which is incorrect. The issue is not revealed in existing UT since current cases didn't check snapshot after state schema migration.

This is a regression issue caused by the FLINK-10043 refactoring work.