Summary:
When ListViews has multiple sections, then rowIDs are required to be unique per section.
However the ListViewMock required rowIDs to be unique per whole list.
This change fixes the limitation of ListViewMock.
Closes https://github.com/facebook/react-native/pull/13642
Differential Revision: D5189675
Pulled By: shergin
fbshipit-source-id: 8a4fb68feb74af3307407d6d70f3f6642f50452a