[ReactNative] prevent ScrollView content offset from changing during layout

Summary:
-[UIScrollView setFrame:] changes the content offset, which we want to do manually.  Preserve the old offset during layout.

React/Views/RCTScrollView.m

@@ -461,7 +461,10 @@ RCT_NOT_IMPLEMENTED(-initWithCoder:(NSCoder *)aDecoder)
   [super layoutSubviews];
   RCTAssert(self.subviews.count == 1, @"we should only have exactly one subview");
   RCTAssert([self.subviews lastObject] == _scrollView, @"our only subview should be a scrollview");
+
+  CGPoint originalOffset = _scrollView.contentOffset;
   _scrollView.frame = self.bounds;
+  _scrollView.contentOffset = originalOffset;
 
   [RCTView autoAdjustInsetsForView:self
                     withScrollView:_scrollView