Add support for value listener

Summary:
Adds support for `Animated.Value#addListener` for native driven animated values. Same as #8844 but for iOS. This depends on some JS code in #8844 so only review the 2nd commit and let's wait for #8844 to land first.

**Test plan**
Tested using the UIExplorer example.
Closes https://github.com/facebook/react-native/pull/9194

Differential Revision: D3681749

fbshipit-source-id: 521a61e2221c1ad1f6f40c75dd2dc957361d0271

Libraries/NativeAnimation/Nodes/RCTValueAnimatedNode.h

@@ -10,8 +10,17 @@
 #import "RCTAnimatedNode.h"
 #import <UIKit/UIKit.h>
 
+@class RCTValueAnimatedNode;
+
+@protocol RCTValueAnimatedNodeObserver <NSObject>
+
+- (void)animatedNode:(RCTValueAnimatedNode *)node didUpdateValue:(CGFloat)value;
+
+@end
+
 @interface RCTValueAnimatedNode : RCTAnimatedNode
 
 @property (nonatomic, assign) CGFloat value;
+@property (nonatomic, weak) id<RCTValueAnimatedNodeObserver> valueObserver;
 
 @end

Libraries/NativeAnimation/Nodes/RCTValueAnimatedNode.m

@@ -11,4 +11,13 @@
 
 @implementation RCTValueAnimatedNode
 
+- (void)setValue:(CGFloat)value
+{
+  _value = value;
+
+  if (_valueObserver) {
+    [_valueObserver animatedNode:self didUpdateValue:_value];
+  }
+}
+
 @end