Add extractOffset to Animated

Summary:
`flattenOffset` has proven extremely useful, especially when dealing with pan responders and other gesture based animations, but I've also found a number of use cases for the inverse. This diff introduces `extractOffset`, which sets the offset value to the base value, and resets the base value to zero. A common use case would be to extractOffset onGrant and flattenOffset onRelease.
Closes https://github.com/facebook/react-native/pull/10721

Differential Revision: D4145744

fbshipit-source-id: dc2aa31652df0b31450556f611db43548180c7dd

ReactAndroid/src/main/java/com/facebook/react/animated/NativeAnimatedModule.java

@@ -262,6 +262,16 @@ public class NativeAnimatedModule extends ReactContextBaseJavaModule implements
     });
   }
 
+  @ReactMethod
+  public void extractAnimatedNodeOffset(final int tag) {
+    mOperations.add(new UIThreadOperation() {
+      @Override
+      public void execute(NativeAnimatedNodesManager animatedNodesManager) {
+        animatedNodesManager.extractAnimatedNodeOffset(tag);
+      }
+    });
+  }
+
   @ReactMethod
   public void startAnimatingNode(
       final int animationId,

ReactAndroid/src/main/java/com/facebook/react/animated/NativeAnimatedNodesManager.java

@@ -157,6 +157,15 @@ import javax.annotation.Nullable;
     ((ValueAnimatedNode) node).flattenOffset();
   }
 
+  public void extractAnimatedNodeOffset(int tag) {
+    AnimatedNode node = mAnimatedNodes.get(tag);
+    if (node == null || !(node instanceof ValueAnimatedNode)) {
+      throw new JSApplicationIllegalArgumentException("Animated node with tag " + tag +
+        " does not exists or is not a 'value' node");
+    }
+    ((ValueAnimatedNode) node).extractOffset();
+  }
+
   public void startAnimatingNode(
     int animationId,
     int animatedNodeTag,

ReactAndroid/src/main/java/com/facebook/react/animated/ValueAnimatedNode.java

@@ -40,6 +40,11 @@ import javax.annotation.Nullable;
     mOffset = 0;
   }
 
+  public void extractOffset() {
+    mOffset += mValue;
+    mValue = 0;
+  }
+
   public void onValueUpdate() {
     if (mValueListener == null) {
       return;