Will the radar animate within ForeFlight using a Garmin Flight Stream and GDL84/88?

Yes, the radar will animate when receiving ADS-B radar from a Garmin GDL 84/88 through the Flight Stream unit.  You will need, however, to leave ForeFlight Mobile powered on with the Radar layer selected on the Maps page to get the radar data since past radar data is not saved by the Flight Stream.  As new ADS-B radar frames are received every 5-15 minutes, you should see the length of the animation loop increase up to a maximum of 30 minutes.