Prevent keyboard from hiding when overlay is shown (#4778)

If you use overlays to show in-app notifications
it would closes the keyboard every time an overlay is
opened.

lib/ios/RNNOverlayManager.m

 	[_overlayWindows addObject:overlayWindow];
 	overlayWindow.rootViewController.view.backgroundColor = [UIColor clearColor];
 	[overlayWindow setWindowLevel:UIWindowLevelNormal];
-	[overlayWindow makeKeyAndVisible];
+	[overlayWindow setHidden: NO];
 }
 
 - (void)dismissOverlay:(UIViewController*)viewController {