American Airlines and the Assn. of Professional Flight Attendants said they have accepted an offer from the National Mediation Board to mediate their contract negotiations. The union is asking its members for authority to call a brief strike--lasting from one to three days--sometime during the Christmas holiday. Previously, American had brushed off the strike threat and said that its operations would not be affected. An official of the airline called involvement by the National Mediation Board "in the interests of both the carrier and APFA."