Fix window.requestIdleCallback not firing on iOS (#29895)

Summary:
Fixes #28602

When creating a `RCTFrameUpdate`, ensure it is created with the correct unix timestamp in seconds. This is needed to match the `NSTimeInterval` type defined in the header.

Previously, it was using the `CADisplayLink`'s `timestamp` property, which is not an `NSTimeInterval` but is instead a `CFTimeInterval` (note the different class prefix). This is the host time converted to seconds, which is the number of seconds since the device was turned on and therefore not a unix timestamp as expected.

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. The code does this calculation is here:

https://github.com/facebook/react-native/blob/4d920fe7c991eaec61d229a71df30b0f6c446d38/React/CoreModules/RCTTiming.mm#L262

As one of these is a valid unix timestamp and the other is a much smaller number (number of seconds since device turn on), the `if` statement following this calculation never passes and the callbacks are never called.

This regression seems to have been introduced with this pull request: #26114

## Changelog

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[iOS] [Fixed] - Fixed window.requestIdleCallback not firing on iOS

Pull Request resolved: #29895

Test Plan: I have tested this by patching my React Native code and testing that idle callbacks are correctly called. There is a reproduction case of the issue in the linked issue.

Reviewed By: NickGerleman

Differential Revision: D47381404

Pulled By: sammy-SC

fbshipit-source-id: fd166741889b0084e1def8dedf6e4018adfd570f

packages/react-native/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];