Reduce visibility of DecayAnimation

Summary:
In an attempt to reduce footprint of React Native Android public APIs we are reducing visibility of classes and interfaces that are not meant to be used publicly OR are public but have no usages.
As part of our analysis, which involved looking for usages inside the Meta codebase and code search in OSS, we've detected that this class/interface is public but it's not used from other packages.

If you are using this class or interface please comment in this PR and we will restate the public access.

changelog: [Android][Changed] Reducing visibility of DecayAnimation

Reviewed By: javache, RSNara

Differential Revision: D49752144

fbshipit-source-id: af0cba34730bd6a86211fed0248e3642fa17f525

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/animated/DecayAnimation.java

@@ -13,7 +13,7 @@
  * Implementation of {@link AnimationDriver} providing support for decay animations. The
  * implementation is copied from the JS version in {@code AnimatedImplementation.js}.
  */
-public class DecayAnimation extends AnimationDriver {
+class DecayAnimation extends AnimationDriver {
 
   private final double mVelocity;