RNTester: Add Animated Loop Example

Summary:
Creates a new `Animated.loop` example in RNTester that uses the native driver.

This example shows the precise problem fixed by facebook/react-native#29585 (D28383538 (https://github.com/facebook/react-native/commit/129180c77b0b99a3acedbeb04ce6ec4667f74cac)).

Changelog:
[Internal]

Reviewed By: kacieb

Differential Revision: D28406914

fbshipit-source-id: 63ea7799d4b8bef8b0c1caaa3daf514ca04b7ab1