[ReactNative] Properly pause frame update observers when idle

Summary:
@public

`RCTDispatchEvent` and `RCTTiming` weren't being paused when there wasn't any
work left to be done.

Test Plan:
Run the timers example - check everything still works as expected
Test the ListView paging example - check scroll events are still fired as expected
Launch UIExplorer, let it idle, and put a break point on `-[RCTBridge dispatchBlock:moduleID:]`,
it should never fire.

React/Base/RCTEventDispatcher.m

@@ -72,6 +72,7 @@ static NSNumber *RCTGetEventID(id<RCTEvent> event)
 }
 
 @synthesize bridge = _bridge;
+@synthesize paused = _paused;
 
 RCT_EXPORT_MODULE()
 
@@ -146,6 +147,7 @@ RCT_IMPORT_METHOD(RCTEventEmitter, receiveEvent);
   }
 
   _eventQueue[eventID] = event;
+  _paused = NO;
 
   [_eventQueueLock unlock];
 }
@@ -180,6 +182,7 @@ RCT_IMPORT_METHOD(RCTEventEmitter, receiveEvent);
   [_eventQueueLock lock];
   eventQueue = _eventQueue;
   _eventQueue = [[NSMutableDictionary alloc] init];
+  _paused = YES;
   [_eventQueueLock unlock];
 
   for (id<RCTEvent> event in eventQueue.allValues) {

React/Modules/RCTTiming.m

@@ -155,6 +155,10 @@ RCT_IMPORT_METHOD(RCTJSTimers, callTimers)
   if ([timersToCall count] > 0) {
     [_bridge enqueueJSCall:@"RCTJSTimers.callTimers" args:@[timersToCall]];
   }
+
+  if (_timers.count == 0) {
+    [self stopTimers];
+  }
 }
 
 /**