Bug with AutoSim and DropHeldObject

Hey @mattdeitke, thanks for the quick turn-around. New issue since I don't have the privileges to re-open https://github.com/allenai/ai2thor/issues/861

Unless I am missing something, I don't think this fixes the problem.. As you can see in this [colab](https://colab.research.google.com/drive/1rKdgrrZkh6mInFyBnTVzZZGCNeHGywpS#scrollTo=cShwrtGjUpnj&line=1&uniqifier=1), this will not work because as soon as the object is dropped using "DropHeldObject", isSceneAtRest becomes True. And so stepping does not work (also I tried getting individual frames and it does not work..)