[EuiFilePicker] Disabled FilePicker shows underlying html input

[crob611]

On the Eui Website

In Canvas

Setting the prop disabled to true disables the html input property. Looks like there is a CSS rule input:disabled {opacity: 1} that is getting precedent over the Filepicker rule that sets it's opacity to 0

[cchaos]

Fun! This happened when trying to fix browser's changing opacity of all disabled inputs. #4538 Terribly, that reset selector is overriding our opacity of the background input.