They won't dry in a plastic bag.
Use a food dehydrator, screens, or an air-hocky table. There are lots of posts in this forum that detail different ways to dry hops. Personally, I like to use a food dehydrator (on the lowest setting) over night. Then I let them sit for a few hours before vacuum sealing and freezing. This method dries them out all the way and then lets them absorb a bit of the ambient moisture. It seems to get them just right. Dry, they should weight 1/4 - 1/5 of the original.
They are perfectly safe for dry hopping. Commercial hop growers don't do anything special to the hops you buy in the store.