A Franconia Township teenager was in juvenile court Friday trying to explain his role in the ethnic intimidation of a Jewish family that lives in his neighborhood.   18-year-old Shane Patrick Stewart of Anjou Court told the judge he thought it was cool to spray paint swastikas and ethnic slurs on the house.   Judge Wendy Demchik-Alloy was incredulous.   This is about as hateful and racist as it comes she said.   Stewart was ruled delinquent and will remain in custody at the Montgomery County Youth Detention Center until sentenced.