Material controls, easy fix to correct the margin of controls when hide the fs button

Hi, thanks for the plugin!
I see that you have added a way to avoid the fullscreen button, which is great for me.
There is only one small detail in android, when the fullscreen button is not shown, the sound button remains with the margin very close to the right edge, it is just an aesthetic detail.

This can be solved by replacing this part of the code in material_controls.dart (line 109).

```
chewieController.allowFullScreen
? _buildExpandButton()
: Container(margin: EdgeInsets.only(right: 12.0),), // add the margin
```

In this way we add the right margin that is lost by removing the fullscreen button.