I've tried a few apps in the Play Store (Twonky, Honey Player) and nothing really works for what you are looking to do unfortunately.
You could do this from a PC or Mac with Airfoil
, but I don't think anyone has developed an app that works consistently to do this. I've used Airfoil on my mediaserver and run Pandora through to an Airport Express successfully so I would imagine it works with your Airplay AVR.
Another cheaper option would be simply running a 3.5mm to RCA from the phone into the AVR and just running the Pandora app. Not ideal, but it does the job.