accessibility bug fix in NumberPicker

* moved View#isVisibleToUser to protected with @hide
* added a new View#isVisibleToUser(Rect), so that a portion of
  the view can be tested for visibility
* NumberPicker will report its concrete class name
* code to append virtual children was at wrong place
* boundInScreen for increment and decrement buttons are reported
  wrong

Change-Id: Ic5d644b3e1efa15b1f0537907c8cdd4ce43a97c1