With Mayor Out, the People Move In; New York Finds Many Uses for Gracie Mansion
Mayor Michael R Bloomberg's decision not to move into Gracie Mansion, home of every New York City mayor since Fiorello H La Guardia, has transformed it into bustling retreat for scores of municipal workers, celebrators, schoolchildren and elected officials; it has housed traveling dignitaries and been used as quite hideaway where officials can conduct city business away from prying eyes at City Hall; mansion has undergone $7 million renovation, with Bloomberg paying most of cost; Bloomberg has also expanded areas open to public and increased number of mansion tours; mansion is still used for dinners and receptions; photos (M)