The National Basketball Association Inc.’s plan to tap African fans and new players in the world’s most youthful continent spurred billionaire Prem Watsa-backed private equity firm to invest in the franchise at a valuation of $1 billion. Helios Fairfax Partners Corp. took a small stake in the U.S. sport’s African entity last year — at first glance a curious choice for a firm that has broadly focused on telecoms through Helios Towers Plc or gas stations via Vivo Energy Plc.