In the latest step in pulling back from its adventure in content and media businesses, AT&T finally offloaded its failed advertising arm, Xandr, just as the world shut down for Christmas. Microsoft is the new parent of the loss-making subsidiary, subject to regulatory approval. The short press release made no mention of financial terms, but it is no secret that AT&T has been trying to sell Xandr for some time, and the price is unlikely to have been hefty. But Microsoft will be hoping that, by placing its audience intelligence and vast advertising customer base within Xandr’s data-driven platform, it will be able to breathe new life into its digital advertising and retail media products. Looking at Microsoft’s existing portfolio,…