[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/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];
+  }
 }
 
 /**