A trained police dog is going to take you down regardless of what you do. They're very intelligent, loyal animals, and when they obey orders they believe they are defending and protecting their master. They will kill you if they think they are being told to. They will most likely interpret the scream as a threat to their pack, which will only make matters worse for you.

Now, a common stray or house dog may act differently. That's really a toss up.
We had a dog who loved and would go home with anyone. Dumb as a box of hammers. I was wrestling with a friend once, and he bit me trying to protect my friend. Weirdo. We had another dog that would relentlessly attack people and animals if he thought our kids were in danger. Cops, fellow marines, didn't care- he was on it. No barking, growling, anything. Without warning, all you would hear is his teeth gnashing while he charged. R.I.P. buddy.