Mobile-application developers are getting advice from firms that specialize in analyzing games and other applications to see what makes them engaging or useful, Olga Kharif writes. The field is small but growing rapidly. Atlantic Bridge, Intel Capital and others last month invested $6.25 million in Swrve, which advises Activision Blizzard and other clients. Another app analytics firm, App Annie, got $6 million in private funding earlier this year.