A bombing threat Saturday night interrupted the first Washington concert in 16 years by the national choir of Israel, Rinat, at the John F. Kennedy Center for the Performing Arts. But after an interval of some 35 minutes that included the regular intermission period, the choir rewarded the approximately 1000 people in the audience with three rousing encores. During its concert, the choir broke away from its published program to render an Arabic song from Lebanon in observance of the Camp David accords. Its encores included a Negro spiritual and two Hebrew songs. The choir, consisting of 22 women and 18 men, is completing its 18th concert tour with programs this week at the 92nd Street YMHA in New York, and in Ottawa, Altoona, Pa. and Peekskill, N.Y.