Fix RCTPerfMonitor not showing up in scene based app (#43476)

Summary:
Currently RCTPerfMonitor won't show up in scene based app, we should first try to extract the window from the connected scenes, and fallback to the window in `UIApplicationDelegate`.

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[iOS] [Fixed] - Fix RCTPerfMonitor not showing up in scene based app

Pull Request resolved: #43476

Test Plan:
- Tested RCTPerfMonitor in app not using scenes;
- Tested RCTPerfMonitor in app using scenes in iOS 13 & 14;
- Tested RCTPerfMonitor in app using scenes in iOS 15+.

Reviewed By: rshest

Differential Revision: D57381551

Pulled By: javache

fbshipit-source-id: fd6cce20c9a4ed41d7aae84751fc0c83391d0865

packages/react-native/React/CoreModules/RCTPerfMonitor.mm

@@ -298,8 +298,7 @@ - (void)show
 
   [self updateStats];
 
-  UIWindow *window = RCTSharedApplication().delegate.window;
-  [window addSubview:self.container];
+  [RCTKeyWindow() addSubview:self.container];
 
   _uiDisplayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(threadUpdate:)];
   [_uiDisplayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];