It shall be unlawful for any person to willfully listen by means of any radio receiving device located in or upon any vehicle to any official message which is being transmitted by the Police Department or Fire Department of the City of Los Angeles or any law enforcement agency over a radio transmitting station owned or operated by such city or agency. The provisions herein shall not apply to any person to whom a permit to listen to such radio messages has been issued in writing by the Chief of Police of the City of Los Angeles after he determines that public interest will be served by the issuance of such permit, nor shall the provisions of this section apply to any officer, agent, or servant of any government agency or public utility, the performance of whose duty as such officer, agent or servant, requires that he listen to such messages.