endRefreshing animates scrollView to top inset instead of zero

Summary:To use a ScrollView and RefreshControl with a translucent navigation bar you have to set the top inset to the height of that bar, allowing the content to scroll underneath. After changes to RCTRefreshControl in  **v0.22**, `endRefreshing` always animates the offset to 0, hiding content behind the navigation bar. What you'd expect on iOS is for it to return to the bottom of the bar.

**Test plan**
To see this in action, refer to the UIExplorerApp. In RefreshControlExample.js if you set the ScrollView's `contentInset={{top: 100}}` you'll see the refresh control UI is where you'd expect, and after refresh the list returns to the correct position.
Closes https://github.com/facebook/react-native/pull/6848

Differential Revision: D3157934

Pulled By: mkonicek

fb-gh-sync-id: c2186a4541fb3988677f0851eb12c259cd003750
fbshipit-source-id: c2186a4541fb3988677f0851eb12c259cd003750