Fix CalendarDatePicker.Focus() not focusing the text box (#22170)

* Fix CalendarDatePicker.Focus() not focusing the text box

OnGotFocus only forwarded focus to PART_TextBox when the navigation method
was Tab, so a programmatic Focus() call left focus on the picker itself.
Gate on the focus source instead, and keep select-all on Tab only.

* Remove comment from CalendarDatePicker.OnGotFocus