Summary:Fixes an issue where if you implement `renderScrollComponent` and have a `ref` callback on the returned element, the ref used to be clobbered by the ref that ListView adds to the element.
This is accomplished by converting the ref from a legacy string-based ref to a callback-based ref, and then using `cloneReferencedElement`, which is a simple utility to compose callback refs.
Closes https://github.com/facebook/react-native/pull/6441
Differential Revision: D3064250
Pulled By: mkonicek
fb-gh-sync-id: 2d55d04e2144a1cc08900a57a1fc0dab07c87eea
fbshipit-source-id: 2d55d04e2144a1cc08900a57a1fc0dab07c87eea
eaba2abc0b