Customizable controls' opacity doesn't go below 15

New customizable controls opacity don't go below 15 so we can't fully customise the opacity to 0.

Steps to reproduce:

1. Create/open a world
2. Enable Customize Controls (Experimental)
3. Go to Customize controls
4. Select a button -> Opacity, try to bring it down to 0%

Observed result:
You cannot bring opacity down to 0%, the minimum allowed value is 15%.

Expected result:
The minimum allowed opacity value is 0%.