[ReactNative] Use NSDictionary + NSNumber for event coalescing

Summary:
@public

On D2099270 event coalescing was implemented and the event key on the RCTSparseArray
is an uint64_t, but it was declared as NSUInteger.  On a 32 bits architecture
it'll be clipped to 4 bits, meaning that just `reactTag` will be taken into
account, e.g. different types of events can coalesce with each other if they target the same view

Switching to use an NSMutableDictionary instead of RCTSparseArray and NSNumber
as keys instead of uint64_t

Test Plan: Fixed the previous tests and added a new test to RCTEventDispatcherTests

React/Base/RCTEventDispatcher.m

@@ -11,11 +11,10 @@
 
 #import "RCTAssert.h"
 #import "RCTBridge.h"
-#import "RCTSparseArray.h"
 
-static uint64_t RCTGetEventID(id<RCTEvent> event)
+static NSNumber *RCTGetEventID(id<RCTEvent> event)
 {
-  return (
+  return @(
     [event.viewTag intValue] |
     (((uint64_t)event.eventName.hash & 0xFFFF) << 32)  |
     (((uint64_t)event.coalescingKey) << 48)
@@ -68,7 +67,7 @@ static uint64_t RCTGetEventID(id<RCTEvent> event)
 
 @implementation RCTEventDispatcher
 {
-  RCTSparseArray *_eventQueue;
+  NSMutableDictionary *_eventQueue;
   NSLock *_eventQueueLock;
 }
 
@@ -79,7 +78,7 @@ RCT_EXPORT_MODULE()
 - (instancetype)init
 {
   if ((self = [super init])) {
-    _eventQueue = [[RCTSparseArray alloc] init];
+    _eventQueue = [[NSMutableDictionary alloc] init];
     _eventQueueLock = [[NSLock alloc] init];
   }
   return self;
@@ -139,7 +138,7 @@ RCT_IMPORT_METHOD(RCTEventEmitter, receiveEvent);
 
   [_eventQueueLock lock];
 
-  uint64_t eventID = RCTGetEventID(event);
+  NSNumber *eventID = RCTGetEventID(event);
   id<RCTEvent> previousEvent = _eventQueue[eventID];
 
   if (previousEvent) {
@@ -176,14 +175,14 @@ RCT_IMPORT_METHOD(RCTEventEmitter, receiveEvent);
 
 - (void)didUpdateFrame:(RCTFrameUpdate *)update
 {
-  RCTSparseArray *eventQueue;
+  NSDictionary *eventQueue;
 
   [_eventQueueLock lock];
   eventQueue = _eventQueue;
-  _eventQueue = [[RCTSparseArray alloc] init];
+  _eventQueue = [[NSMutableDictionary alloc] init];
   [_eventQueueLock unlock];
 
-  for (id<RCTEvent> event in eventQueue.allObjects) {
+  for (id<RCTEvent> event in eventQueue.allValues) {
     [self dispatchEvent:event];
   }
 }