Revert the changes to idle timers made in facebook#26114

This was causing issues with the window.requestIdleCallback timers as the timer code was expecting this "timestamp" property to be a unix timestamp in seconds which was causing the calculations to be done incorrectly and for the callbacks to never be called.

Fixes facebook#28602

React/CoreModules/RCTTiming.mm

@@ -256,9 +256,9 @@ - (void)didUpdateFrame:(RCTFrameUpdate *)update
   }
 
   if (_sendIdleEvents) {
-    NSTimeInterval currentTimestamp = [[NSDate date] timeIntervalSince1970];
-    NSTimeInterval frameElapsed = currentTimestamp - update.timestamp;
+    NSTimeInterval frameElapsed = (CACurrentMediaTime() - update.timestamp);
     if (kFrameDuration - frameElapsed >= kIdleCallbackFrameDeadline) {
+      NSTimeInterval currentTimestamp = [[NSDate date] timeIntervalSince1970];
       NSNumber *absoluteFrameStartMS = @((currentTimestamp - frameElapsed) * 1000);
       if (_bridge) {
         [_bridge enqueueJSCall:@"JSTimers" method:@"callIdleCallbacks" args:@[ absoluteFrameStartMS ] completion:NULL];