The former Performing Arts Center in downtown Lansdale was the focus of a public meeting Tuesday night.   A-M-S Planning and Research, the consultant hired by borough council to help determine a direction for the building, divided the 50 or so meeting participants into five groups that came up with several potential uses for the old Arts Center.   Those ideas will be looked at closely to see how practical they are.   A-M-S Planning and Research will be paid almost $49,000 to come up with a direction and a business plann for the borough owned building on West Main Street.   Lansdale officials expect a report in early fall.