Fix focus set for delegated and already focused elements (#36010)

I found two focus bugs when working on documentation for Fragment Refs.

1) If an element delegates focus handling, it will return false from
setFocusIfFocusable even though a focus event has occured on a different
element. The fix for this is a document level event listener rather than
only listening on the current element.

For example, if you have a form with multiple nested label>inputs.
Calling focus on the label will focus its input but not fire an event on
the label. setFocusIfFocusable returns false and you end up continuing
to attempt focus down the form tree.

2) If an element is already focused, setFocusIfFocusable will return
false. The fix for this is checking the document's activeElement with an
early return.

In the same form example, if the first input is already focused and you
call fragmentInstance.focus() at the form level, the second input would
end up getting focused since the focus event on the first is not
triggered.