Fixed regression in SectionList caused by #21577 not being able to scroll to top on android (#24034)

Summary:
```
let index = params.itemIndex + 1;
```
to
```
let index = Platform.OS === 'ios' ? params.itemIndex : params.itemIndex - 1;
```
to fix an issue on iOS. Note however, how the sign for non iOS changed from `+` to `-` causing a crash on Android when trying to scroll to index 0 as that will be evaluated to -1 instead of 1 and thus be out of bounds.

[Android] [Fixed] - Fixed regression in SectionList caused by #21577 not being able to scroll to top on android
Pull Request resolved: https://github.com/facebook/react-native/pull/24034

Differential Revision: D14520796

Pulled By: cpojer

fbshipit-source-id: bb49619f49752fd3f343ef3b7bf1b86ac48af7f8

Libraries/Lists/VirtualizedSectionList.js

@@ -146,7 +146,7 @@ class VirtualizedSectionList<SectionT: SectionBase> extends React.PureComponent<
     sectionIndex: number,
     viewPosition?: number,
   }) {
-    let index = Platform.OS === 'ios' ? params.itemIndex : params.itemIndex - 1;
+    let index = Platform.OS === 'ios' ? params.itemIndex : params.itemIndex + 1;
     for (let ii = 0; ii < params.sectionIndex; ii++) {
       index += this.props.sections[ii].data.length + 2;
     }