S-1 Corp. is a small part of Samsung’s sprawling business empire. But an activist investor’s bid for Samsung affiliates’ stakes in the security company poses the question: When should a conglomerate’s companies sell their holdings in one another? Singapore-based Flashlight Capital Partners brought that question into focus in August, offering 906.6 billion won ($675 million) for the S-1 shares held by five Samsung affiliates: Samsung SDI, Samsung Life Insurance, Samsung Fire & Marine Insurance, S