[Shared Element Transition] Fix: view disappeared after layout animat…

…ion (software-mansion#4026)

## Summary

I added a check to prevent adding already existing views to the shared
transition registry. The issue only occurred when the view had a layout
animation, as the registry is cleared after the animation ends.
Afterwards, views from the registry were removed from their parent view.
In the case of a shared transition, the parent screen is a custom
temporary transition container, but in the case of a regular layout
animation, the super view is the original parent. As a result, the view
disappeared.

ios/LayoutReanimation/REAAnimationsManager.m

@@ -521,7 +521,7 @@ - (void)viewDidMount:(UIView *)view withBeforeSnapshot:(nonnull REASnapshot *)be
     [self setNewProps:before.values forView:view];
   }
 
-  if (_hasAnimationForTag(viewTag, @"sharedElementTransition")) {
+  if (_hasAnimationForTag(viewTag, @"sharedElementTransition") && [type isEqual:@"entering"]) {
     [_sharedTransitionManager notifyAboutNewView:view];
   }
 }

ios/LayoutReanimation/REASharedTransitionManager.m

@@ -75,8 +75,10 @@ - (void)viewsDidLayout
 
 - (void)configureAsyncSharedTransitionForViews:(NSArray<UIView *> *)views
 {
-  NSArray *sharedViews = [self sortViewsByTags:views];
-  _sharedElements = [self getSharedElementForCurrentTransition:sharedViews withNewElements:YES];
+  if ([views count] > 0) {
+    NSArray *sharedViews = [self sortViewsByTags:views];
+    _sharedElements = [self getSharedElementForCurrentTransition:sharedViews withNewElements:YES];
+  }
 }
 
 - (BOOL)configureAndStartSharedTransitionForViews:(NSArray<UIView *> *)views