Hoping no one would notice, AT&T dropped its failed advertising arm Xandr just as the world shut down for Christmas. Microsoft is the hopeful new parent of the loss-making subsidiary, subject to regulatory approval. The question is now whether Microsoft can slim down Xandr to a few core pursuits and relieve it of its identity crisis by sidelining its video arm? The short press release made no mention of financial terms, but it is no secret that AT&T has been trying to flog Xandr for yonks now. When the price tag eventually surfaces in some SEC filings, we expect nothing more than a bargain bucket number. The details are still so slim that it remains unclear whether Microsoft is set…