Replace `ScrollView.scrollTo()` API with something less ambiguous.

Summary:
public
The current `ScrollView.scrollTo()` API is confusing due to the `(y, x)` parameter order, and the boolean `animated` argument. E.g.

    ScrollView.scrollTo(5, 0, true) // what do these arguments mean?

This diff replaces the API with a configuration object, so the arguments are all explicit:

    ScrollView.scrollTo({x: 0, y: 5, animated: true}) // much better

The `scrollTo()` method checks the argument types, and provides backwards compatibility with the old argument format for now. Using the old API will generate a warning, and this will eventually be upgraded to an error.

Reviewed By: davidaurelio

Differential Revision: D2892287

fb-gh-sync-id: cec4d504242391267c6e863816b6180ced7a7d5e