Revert "fix: move onIndexChange (#1384)"

This reverts commit 4616336.

Author: okwasniewski

src/PanResponderAdapter.tsx

@@ -81,8 +81,6 @@ export default function PanResponderAdapter<T extends Route>({
 
       const { timing, ...transitionConfig } = DefaultTransitionSpec;
 
-      onIndexChangeRef.current(index);
-
       Animated.parallel([
         timing(panX, {
           ...transitionConfig,
@@ -91,6 +89,7 @@ export default function PanResponderAdapter<T extends Route>({
         }),
       ]).start(({ finished }) => {
         if (finished) {
+          onIndexChangeRef.current(index);
           pendingIndexRef.current = undefined;
         }
       });