Fix memory leak issue by explicitly clean up contentRef upon unmount

[FanchenBao]

I encounter this warning while using the auto scrolling component ("react": "^17.0.1", "react-native": "^0.64.0").

Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.

It typically means some async function is called after AutoScrolling is unmounted. The culprit points to setIsAutoScrolling inside checkContent. This is understandable as checkContent is called in measureContainerView, which is a callback that can be scheduled to run after component unmount.

The fix is to clean up contentRef, such that checkContent won't be called in measureContainerView upon component unmount.

[minhtc]

Thanks for your pull request