Construct the shared animation backend after the UIManager has a delegate (#57817)

Summary:
Pull Request resolved: #57817

## Changelog:

[Internal] - Construct the shared animation backend after the UIManager has a delegate

`AnimationBackend`'s constructor calls `UIManager::addOnSurfaceStartCallback` so that `AnimatedPropsRegistry::initializeSurface` runs for each surface as it starts. That call is a no-op unless the `UIManager` already has a delegate:

```
void UIManager::addOnSurfaceStartCallback(
    UIManagerDelegate::OnSurfaceStartCallback&& callback) {
  if (delegate_ != nullptr) {
    delegate_->uiManagerShouldAddOnSurfaceStartCallback(std::move(callback));
  }
}
```

`Scheduler`'s constructor was building the backend immediately after constructing the `UIManager` and roughly seventy lines before `uiManager->setDelegate(this)`, so the callback was dropped every time and `Scheduler::onSurfaceStartCallbacks_` never received it. `AnimatedPropsRegistry::update` skips surfaces missing from `surfaceContexts_`, so correctness was left resting on `getMap()` default-constructing the entry via `operator[]` — that is, on a commit hook happening to run before the first animated update on a freshly started surface. When that ordering does not hold, early animated updates on a new surface are dropped.

Move the backend construction to just after `setDelegate`, and add a comment recording the ordering constraint. This is still well before any surface can start, so the registry is populated for every surface from the first frame.

The neighbouring `getShadowTreeRegistry().enumerate(...)` in the same constructor is also dead today — the `UIManager` is two lines old and its registry is necessarily empty — but it is left in place deliberately. It mirrors the enumerate-then-register pattern in `NativeAnimatedNodesManagerProvider` and `ViewTransitionModule`, both of which are constructed lazily and do need it, and it keeps `AnimationBackend`'s constructor correct if it is ever moved to a lazy call site.

Reviewed By: christophpurrer

Differential Revision: D114759538

fbshipit-source-id: f3eeab96d2ca1ada4feaf85d49a3f0ac169f3115

Author: zeyap

packages/react-native/ReactCommon/react/renderer/scheduler/Scheduler.cpp

@@ -59,16 +59,6 @@ Scheduler::Scheduler(
   auto uiManager =
       std::make_shared<UIManager>(runtimeExecutor_, contextContainer_);
 
-  if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
-    auto animationBackend = std::make_shared<AnimationBackend>(
-        schedulerToolbox.animationChoreographer, uiManager);
-
-    schedulerToolbox.animationChoreographer->setAnimationBackend(
-        animationBackend);
-
-    uiManager->unstable_setAnimationBackend(animationBackend);
-  }
-
   auto eventOwnerBox = std::make_shared<EventBeat::OwnerBox>();
   eventOwnerBox->owner = eventDispatcher_;
 
@@ -131,6 +121,19 @@ Scheduler::Scheduler(
   uiManager->setDelegate(this);
   uiManager->setComponentDescriptorRegistry(componentDescriptorRegistry_);
 
+  // Must come after `setDelegate`: the backend's constructor registers a
+  // surface-start callback, and `UIManager` silently drops those while it has
+  // no delegate.
+  if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+    auto animationBackend = std::make_shared<AnimationBackend>(
+        schedulerToolbox.animationChoreographer, uiManager);
+
+    schedulerToolbox.animationChoreographer->setAnimationBackend(
+        animationBackend);
+
+    uiManager->unstable_setAnimationBackend(animationBackend);
+  }
+
   auto bindingsExecutor =
       schedulerToolbox.bridgelessBindingsExecutor.has_value()
       ? schedulerToolbox.bridgelessBindingsExecutor.value()