Moves viewControllerForKey to use viewForKey with transitions where (#6238)

Moves viewControllerForKey to use viewForKey with transitions where appropriate as discussed in this Stack Overflow post: https://stackoverflow.com/questions/24338700/from-view-controller-disappears-using-uiviewcontrollercontexttransitioning/25193675#25193675

This mainly resolves an issue when using animations with `dismissModal` that results in all views being removed and a black screen appearing as discussed in #6195 

The remaining `viewControllerForKey` calls should probably be removed too where their `view` property is being used, but this seems to resolve the bug for now at least!

lib/ios/ModalDismissTransitionDelegate.m

 }
 
 - (void)prepareTransitionContext:(id<UIViewControllerContextTransitioning>)transitionContext {
-    UIViewController* toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
-    UIViewController* fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
+    UIView* toView = [transitionContext viewForKey:UITransitionContextToViewKey];
+    UIView* fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
     
-    fromVC.view.alpha = 0;
-    [transitionContext.containerView addSubview:toVC.view];
-    [transitionContext.containerView addSubview:fromVC.view];
+    fromView.alpha = 0;
+    [transitionContext.containerView addSubview:toView];
+    [transitionContext.containerView addSubview:fromView];
 }
 
 @end

lib/ios/ModalTransitionDelegate.m

 }
 
 - (void)prepareTransitionContext:(id<UIViewControllerContextTransitioning>)transitionContext {
-    UIViewController* toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
-    toVC.view.alpha = 0;
-    [transitionContext.containerView addSubview:toVC.view];
+    UIView* toView = [transitionContext viewForKey:UITransitionContextToViewKey];
+    toView.alpha = 0;
+    [transitionContext.containerView addSubview:toView];
 }
 
 - (nullable id <UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source {

lib/ios/RNNAnimationsTransitionDelegate.m

 }
 
 - (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext {
-	UIViewController* toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
-	UIViewController* fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
+	UIView* toView = [transitionContext viewForKey:UITransitionContextToViewKey];
+	UIView* fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
 	
 	[CATransaction begin];
 	[CATransaction setCompletionBlock:^{
 	}];
 	
 	if (_isDismiss) {
-		[[transitionContext containerView] addSubview:toViewController.view];
-		[[transitionContext containerView] addSubview:fromViewController.view];
-		[self animateElement:self.transitionOptions view:fromViewController.view elementName:@"content"];
+		[[transitionContext containerView] addSubview:toView];
+		[[transitionContext containerView] addSubview:fromView];
+		[self animateElement:self.transitionOptions view:fromView elementName:@"content"];
 	} else {
-		[[transitionContext containerView] addSubview:toViewController.view];
-		[self animateElement:self.transitionOptions view:toViewController.view elementName:@"content"];
+		[[transitionContext containerView] addSubview:toView];
+		[self animateElement:self.transitionOptions view:toView elementName:@"content"];
 	}
 	
 	[CATransaction commit];

lib/ios/RNNPushAnimation.m

 }
 
 - (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext {
-	UIViewController* fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
-	UIViewController* toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
+    UIViewController *toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
+	UIView* fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
+	UIView* toView = [transitionContext viewForKey:UITransitionContextToViewKey];
 	
-	[[transitionContext containerView] addSubview:fromViewController.view];
-	[[transitionContext containerView] addSubview:toViewController.view];
+	[[transitionContext containerView] addSubview:fromView];
+	[[transitionContext containerView] addSubview:toView];
 	
 	[CATransaction begin];
 	[CATransaction setCompletionBlock:^{

lib/ios/TransitionDelegate.m

 }
 
 - (void)prepareTransitionContext:(id<UIViewControllerContextTransitioning>)transitionContext {
-    UIViewController* toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
-    toVC.view.alpha = 0;
-    [transitionContext.containerView addSubview:toVC.view];
+    UIView* toView = [transitionContext viewForKey:UITransitionContextToViewKey];
+    toView.alpha = 0;
+    [transitionContext.containerView addSubview:toView];
 }
 
 - (void)performAnimationOnce {