breaking change: make it clearer when a disabled switch is already checked

[jgravois]

no offense taken if you don't consider this an improvement, but in our own app we differentiate the two components below👇 by swapping the default color back in and applying a partial opacity.

<Switch disabled />
<Switch disabled checked />

before:

after:

[rtivital]

I consider this to be a breaking change. Cannot merge for 8.x releases. I will consider it with 9.0 release.