The company that shook up television with original shows such as “House of Cards” has been aggressively building its overseas presence as growth slows in the United States. Netflix added 2.6 million customers in its nearly 50 international markets in the quarter ended March 31, bringing the total to 62.27 million users worldwide. Customers spent more time than ever watching Netflix, streaming 10 billion hours of programming in the quarter, the company said.