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

Libraries/NativeAnimation/RCTNativeAnimatedModule.h

@@ -7,7 +7,9 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  */
 #import "RCTBridgeModule.h"
+#import "RCTValueAnimatedNode.h"
+#import "RCTEventEmitter.h"
 
-@interface RCTNativeAnimatedModule : NSObject <RCTBridgeModule>
+@interface RCTNativeAnimatedModule : RCTEventEmitter <RCTBridgeModule, RCTValueAnimatedNodeObserver>
 
 @end

Libraries/NativeAnimation/RCTNativeAnimatedModule.m

@@ -32,13 +32,12 @@
   CADisplayLink *_displayLink;
 }
 
-@synthesize bridge = _bridge;
-
 RCT_EXPORT_MODULE()
 
 - (void)setBridge:(RCTBridge *)bridge
 {
-  _bridge = bridge;
+  [super setBridge:bridge];
+
   _animationNodes = [NSMutableDictionary new];
   _animationDrivers = [NSMutableDictionary new];
   _activeAnimations = [NSMutableSet new];
@@ -47,11 +46,17 @@ RCT_EXPORT_MODULE()
   _propAnimationNodes = [NSMutableSet new];
 }
 
+
 - (dispatch_queue_t)methodQueue
 {
   return dispatch_get_main_queue();
 }
 
+- (NSArray<NSString *> *)supportedEvents
+{
+  return @[@"onAnimatedValueUpdate"];
+}
+
 RCT_EXPORT_METHOD(createAnimatedNode:(nonnull NSNumber *)tag
                   config:(NSDictionary<NSString *, id> *)config)
 {
@@ -198,6 +203,29 @@ RCT_EXPORT_METHOD(dropAnimatedNode:(nonnull NSNumber *)tag)
   }
 }
 
+RCT_EXPORT_METHOD(startListeningToAnimatedNodeValue:(nonnull NSNumber *)tag)
+{
+  RCTAnimatedNode *node = _animationNodes[tag];
+  if (node && [node isKindOfClass:[RCTValueAnimatedNode class]]) {
+    ((RCTValueAnimatedNode *)node).valueObserver = self;
+  }
+}
+
+RCT_EXPORT_METHOD(stopListeningToAnimatedNodeValue:(nonnull NSNumber *)tag)
+{
+  RCTAnimatedNode *node = _animationNodes[tag];
+  if (node && [node isKindOfClass:[RCTValueAnimatedNode class]]) {
+    ((RCTValueAnimatedNode *)node).valueObserver = nil;
+  }
+}
+
+- (void)animatedNode:(RCTValueAnimatedNode *)node didUpdateValue:(CGFloat)value
+{
+  [self sendEventWithName:@"onAnimatedValueUpdate"
+                     body:@{@"tag": node.nodeTag, @"value": @(value)}];
+}
+
+
 #pragma mark -- Animation Loop
 
 - (void)startAnimation