The investment duo slated to buy the Sony building in Midtown for $1.1 billion purchased a shuttered luxury hotel in Midtown earlier this month, a 46-story property they reportedly plan to convert to high-end condos.   Investors Joseph Chetrit and David Bistricer bought the Flatotel, on West 52nd Street near Seventh Avenue, for about $180 million in early February, according to a report by Crain's. The hotel, which housed 289 rooms and Moda Restaurant, was foreclosed upon by three investment groups after the building's owner, the Alexico Group, defaulted on debts against the property.