In #31614, I added an unfocus() to FocusNodes to allow giving up of focus, but it only worked on the primary focus. This changes that so that it will unfocus the entire chain, not just the primary focus. Now, if you call unfocus() on a FocusNode or FocusScopeNode, and their hasFocus returns true, then after calling unfocus(), it will return false. Before this change, it would only do that if hasPrimaryFocus was also true.

This also fixes a bug in the way setFirstFocus was implemented, making it conform more to the behavior of the previous implementation. It has simplified logic in reparent, and in when it requests focus for scope nodes that have had setFirstFocus called on them.