Optimize RCTUIManager to avoid too many calls to viewForReactTag

Summary:
This change fixes a couple of issues within the RCTUIManager:
* it calls the right method in the `super` branches (although they should neve be executed)
* it invert the call order between the `_registry` and the `uiManager` to avoid extra calls into the `viewForReactTag`.

## Changelog:
[Internal] - Use the right method in super and invert the order of where we search for views.

## Facebook:
See S397861 and T180527210 for more information.

Reviewed By: javache

Differential Revision: D54246220

fbshipit-source-id: 1c7503ad3e80cf50ecc016a984ca180a19b73cc0

Author: cipolleschi

packages/react-native/React/Modules/RCTUIManager.m

@@ -1674,28 +1674,28 @@ - (instancetype)initWithUIManager:(RCTUIManager *)uiManager andRegistry:(NSDicti
 {
   self = [super init];
   if (self) {
-    self->_uiManager = uiManager;
-    self->_registry = registry;
+    _uiManager = uiManager;
+    _registry = registry;
   }
   return self;
 }
 
 - (id)objectForKey:(id)key
 {
   if (![key isKindOfClass:[NSNumber class]]) {
-    return [super objectForKeyedSubscript:key];
+    return [super objectForKey:key];
   }
 
   NSNumber *index = (NSNumber *)key;
-  UIView *view = [_uiManager viewForReactTag:index];
+  UIView *view = _registry[index];
   if (view) {
     return [RCTUIManager paperViewOrCurrentView:view];
   }
-  view = _registry[index];
+  view = [_uiManager viewForReactTag:index];
   if (view) {
     return [RCTUIManager paperViewOrCurrentView:view];
   }
-  return [super objectForKeyedSubscript:key];
+  return [super objectForKey:key];
 }
 
 - (void)removeObjectForKey:(id)key