HONG KONG — For many relatives of the 239 people who were on Malaysia Airlines Flight 370 when it vanished 17 months ago, there was little or no solace in the Malaysian prime minister’s announcement that a wing part found on a remote island in the Indian Ocean had been “conclusively confirmed” to be from the plane.The grief, frustration and suspicion of the authorities that had begun to recede with the passage of time was only rekindled.Many questioned the timing and the motives of the announcement by Prime Minister Najib Razak of Malaysia in the early hours of Thursday, which came before Boeing, the plane’s manufacturer, was prepared to verify that the part was from the missing plane, Flight 370. French investigators would say only that there were “very strong presumptions” that the wing part, called a flaperon, was from the plane, a Boeing 777.