Billionaire investor Warren Buffett told CNBC on Monday his conglomerate Berkshire Hathaway Inc. had more than doubled its stake in Apple Inc. since the end of 2016, making it one of Berkshire’s biggest equity holdings, and that U.S. stocks overall were not in “bubble territory.” “Apple strikes me as having quite a sticky product and an enormously useful product to people that use it, not that I do,” said Buffett, chairman and chief executive of Berkshire Hathaway.