Fixed bug where layoutSubviews was called continuously for scrollview

Summary:
RCTScrollView was calling `dockClosestSectionHeader` in `reactBridgeDidFinishTransaction`, which triggers a layout update. The reason for this was in case the `stickyHeaderIndexes` property was updated, which would require the headers to be adjusted.

However, doing this in `reactBridgeDidFinishTransaction` had the affect of causing `layoutSubviews` to be called repeatedly every frame even if nothing had changed and the scrollview wasn't moving, which was especially expensive when combined with the `removeClippedSubviews` logic, that loops through every view to calculate if it needs to be clipped.

This fix moves the `dockClosestSectionHeader` call into `didUpdateProps`, and checks that the stickyHeaderIndexes have actually changed before calling it.

Reviewed By: javache

Differential Revision: D3387607

fbshipit-source-id: c71e00c6fac48337a63d7fee7c7c23e016acf24e

React/Views/RCTScrollView.m

@@ -894,8 +894,13 @@ - (void)reactBridgeDidFinishTransaction
                  lastIndex, subviewCount);
     }
   }
+}
 
-  [_scrollView dockClosestSectionHeader];
+- (void)didSetProps:(NSArray<NSString *> *)changedProps
+{
+  if ([changedProps containsObject:@"stickyHeaderIndices"]) {
+    [_scrollView dockClosestSectionHeader];
+  }
 }
 
 // Note: setting several properties of UIScrollView has the effect of