Bug 962286 making sdk/window/utils isFocused export a dispatcher which now works for high level windows

I'm not sure how `isFocused` and `isPrivate` can works, here, they shouldn't, 'cause `this` should be `window`– I can imaging that this definition is never used because is override by the `implementation` below.
However, I think it's better fixing this, re-using `getWindowView` would be also less error prone and we'll have less code to handle if we change the underling implementations. So Something like:
```js
isPrivate.define(BrowserWindow, window => isWindowPrivate(getWindowView(window)));
…
isFocused.define(BrowserWindow, window => isFocused(getWindowView(window)));
```