“I know well how devastating it can be,” Andrews said in her first public appearance in three years. “May we all become a beacon of light to stop it.” WASHINGTON — An Arizona conference on Parkinson’s disease was alive with the sound of Julie Andrews this week, as the 90-year-old “Mary Poppins” actress and dame recorded a message for attendees. “I’m Julie Andrews, and I’m pleased to welcome you to the Seventh World Parkinson’s Congress,” the actress said Sunday while appearing on camera at the start of the research forum. This story continues below. “I know well…