Fix black snapshot issue

Summary:
Many users are reporting an issue with a black snapshot occuring on messenger and fbios.

https://fb.workplace.com/groups/flippersupport/permalink/1935441610269821/

This is because the snapshotting code snapshots all windows in order and composites them together. https://fburl.com/code/gcnbkpda

The issue is the app can add additional windows such as this one
{F1916776068}

which itself it full screen and contains an empty view which effetively covers the real content with black pixels. Additionally react native adds a window to display the debug with metro banner

I dont know how in the real app this doesst paint over the real content but for whatever it causes issues with our snapshotting process

The solution is to only snapshot the active child of the application. This is how it works on android.

The theory is that if we are only traversing the key window (active chld means we dont traverse other children) we should also only snapshot that one

Reviewed By: lblasa

Differential Revision: D64044875

fbshipit-source-id: 9ed83a0ef5f89948f1575c09f496384471985a56

iOS/Plugins/FlipperKitUIDebuggerPlugin/FlipperKitUIDebuggerPlugin/Descriptors/UIDUIApplicationDescriptor.mm

@@ -59,7 +59,12 @@ - (UIDBounds*)boundsForNode:(UIApplication*)node {
 }
 
 - (UIImage*)snapshotForNode:(UIApplication*)node {
-  return UIDApplicationSnapshot(node, [self childrenOfNode:node]);
+  NSMutableArray<UIWindow*>* windows = [NSMutableArray new];
+  UIWindow* window = node.keyWindow;
+  if (window != nil) {
+    [windows addObject:window];
+  }
+  return UIDApplicationSnapshot(node, windows);
 }
 
 @end