Let the player decide whether to start preroll right away

Player has to make a similar decision anyway if there's not a preroll. So let's keep that logic just in the playerservice rather than duplicating here
Preroll doesn't need to listen for webkitfullscreen

Used to be (or at least we thought) that iPhone wasn't triggering 'play' events, so we listened for 'webkitbeginfullscreen' to learn if the video began playing. 

We're seeing the 'play' event now, so we can simplify this logic here.