China’s antitrust watchdog is laying the groundwork for a potential probe into Apple Inc.’s policies and the fees it charges app developers, part of a broader push by Beijing that risks becoming another flashpoint in the country’s trade war with the US.The State Administration for Market Regulation is examining Apple’s policies, which include taking a cut of as much as 30% on in-app spending and barring external payment services and stores, people familiar with the matter said. Agency officials ...