Changes in PopupWindow API

Restriced access to private mScrollChangedListener private field and to
private invokePopup method. Removed @hide annotation for several methods
to allow deeper customization of PopupWindow and it's transitions.

Bug: 123768397
Bug: 123769350
Bug: 123769516
Bug: 123768511
Bug: 123768510
Bug: 123768589
Test: Tests has been added to PopupWindowTest in I468b592a3ba2e5f2f895210d21843d328a454794
Change-Id: I7fb4d8e28f0de889e110b7b90f71ebec9ab4cc9d