[amp-story-player] Ability to make the play/pause button visible in non desktop sizes

## Describe the new feature or change to an existing feature you'd like to see

Reading at the documentation of the amp-story-player, it seems like the only way to show the play/pause button in an embedded player is to build your own control. You cannot make the built-in play/pause button to appear.

This seems like extra work for no reason. Even if you build your own, it will appear either at the start of the controls or at the end, which does not feel natural. The built in play/pause usually appears before the share button.

I do realize it shows up when the aspect ration is *desktop* but when using a player to embed on a web page you may choose a mobile like aspect ratio.

## Describe alternatives you've considered

I can write my own custom button to control the play state by listening to events, but it seems like redundant work