Avoid interrupting scrolling in nested scrollviews

Summary:
When we nest scrollviews, the hack in handleCustomPan: will cause the scroll to stall and break.

There's still some weird behaviour inside ScrollResponder.js but this does not seem the affect the scrollview's scrolling.

public

Reviewed By: fionaf, jingc, majak

Differential Revision: D2713639

fb-gh-sync-id: ad898ead62415bc14c91bc84fdfdb8c0fbb32b06

React/Views/RCTScrollView.m

@@ -182,7 +182,7 @@ - (BOOL)_shouldDisableScrollInteraction
 
 - (void)handleCustomPan:(__unused UIPanGestureRecognizer *)sender
 {
-  if ([self _shouldDisableScrollInteraction]) {
+  if ([self _shouldDisableScrollInteraction] && ![[RCTUIManager JSResponder] isKindOfClass:[RCTScrollView class]]) {
     self.panGestureRecognizer.enabled = NO;
     self.panGestureRecognizer.enabled = YES;
     // TODO: If mid bounce, animate the scroll view to a non-bounced position