Fixed ListViewMock unique key error when many sections

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

Libraries/Lists/ListView/__mocks__/ListViewMock.js

@@ -32,9 +32,10 @@ class ListViewMock extends React.Component {
       const rowIDs = allRowIDs[sectionIdx];
       for (let rowIdx = 0; rowIdx < rowIDs.length; rowIdx++) {
         const rowID = rowIDs[rowIdx];
+        // Row IDs are only unique in a section
         rows.push(
           <StaticRenderer
-            key={rowID}
+            key={'section_' + sectionID + '_row_' + rowID}
             shouldUpdate={true}
             render={this.props.renderRow.bind(
               null,