Improve z-index implementation on Android

Summary:
Use `getChildDrawingOrder` instead of reordering views. The old implementation didn't work properly when `removeClippedSubviews` was enabled and this one should have better performance since we don't play with the view hierarchy at all.

This fixes weird bugs with sticky headers in `SectionList` and allows removing the hack that disabled `removeClippedSubviews` when using sticky section headers.

**Test plan**
Tested using the SectionList and ListViewPaging examples that use sticky headers which uses z-index.
Closes https://github.com/facebook/react-native/pull/13105

Reviewed By: sahrens

Differential Revision: D4765869

Pulled By: achen1

fbshipit-source-id: be3c824658a3ce965b6e7324ad95c77cbd8a86ae

Examples/UIExplorer/js/ListViewPagingExample.js

@@ -195,6 +195,7 @@ class ListViewPagingExample extends React.Component {
         initialListSize={10}
         pageSize={4}
         scrollRenderAheadDistance={500}
+        stickySectionHeadersEnabled
       />
     );
   }

Examples/UIExplorer/js/SectionListExample.js

@@ -59,7 +59,7 @@ const VIEWABILITY_CONFIG = {
 };
 
 const renderSectionHeader = ({section}) => (
-  <View>
+  <View style={styles.header}>
     <Text style={styles.headerText}>SECTION HEADER: {section.key}</Text>
     <SeparatorComponent />
   </View>
@@ -144,6 +144,7 @@ class SectionListExample extends React.PureComponent {
           refreshing={false}
           renderItem={this._renderItemComponent}
           renderSectionHeader={renderSectionHeader}
+          stickySectionHeadersEnabled
           sections={[
             {renderItem: renderStackedItem, key: 's1', data: [
               {title: 'Item In Header Section', text: 'Section s1', key: '0'},
@@ -186,6 +187,9 @@ class SectionListExample extends React.PureComponent {
 }
 
 const styles = StyleSheet.create({
+  header: {
+    backgroundColor: '#e9eaed',
+  },
   headerText: {
     padding: 4,
   },