Twentieth Century Fox Film Corp. gave top billing to its summer box-office successes, "Aliens" and "The Fly," in reporting a nearly 300% increase in its net earnings for the fiscal 1987 first quarter ended Sept. 27. At the same time, the Los Angeles-based firm reported it paid only about one-third as much income tax in the latest quarter as it did in last year's first quarter, which a spokeswoman said was because of a major tax credit. In the year-ago period, Fox had reported a $2.2- million net profit instead of an $18.3- million loss, due to a $20.5-million operating loss carryforward. Fox's first quarter the last fiscal year ended on Nov.30, two months later than this one. Fox shortened its 1986 fiscal year to the 10 months ended June 28, putting it in line with the financial periods of Fox's parent company, News Corp. Ltd., which is controlled by financier Rupert Murdoch.