Decide NativeAnimated backend per-instance instead of live flag (#57205)

[zeyap]

Summary:

Changelog:

[Internal][Fixed] - Fix EXC_BAD_ACCESS / KERN_INVALID_ADDRESS in C++ Animated when useSharedAnimatedBackend() flips across a reused runtime

C++ Animated chose between the legacy and shared-AnimationBackend code paths by reading ReactNativeFeatureFlags::useSharedAnimatedBackend() live, in many places. That flag is a process-global singleton (ReactNativeFeatureFlags::accessor_). On some app the global is reset and re-applied on every user switch (FBReactModule setUpReactNativeFeatureFlags -> dangerouslyReset()/override()), and the previous user's runtime is kept alive and reused. The shared AnimationBackend is attached only once, when an instance's Scheduler is constructed, gated on the flag at that moment. On a multi-account device the global flag could therefore read true on a reused instance whose backend was never attached, so getOrCreate took the shared path and dereferenced a null backend. It also let JS and C++ disagree, since JS caches the flag per runtime while C++ followed the mutated global.

Fix: make the per-instance decision once and use it everywhere instead of the live flag.

• NativeAnimatedNodesManagerProvider::getOrCreate selects the path by whether the shared AnimationBackend actually exists for this instance (unstable_getAnimationBackend().lock() != nullptr).
• NativeAnimatedNodesManager stores a const bool useSharedAnimatedBackend_, latched in its constructor (true for the shared-backend ctor, false for the legacy ctor), and exposes it via useSharedAnimatedBackend(). All internal reads now use the member.
• PropsAnimatedNode reads the decision through manager_->useSharedAnimatedBackend().

The attach side (Scheduler) is intentionally unchanged: it remains the single construction-time read that latches the per-instance decision the rest of the code now follows. This keeps JS and C++ consistent across global flag flips and removes the null dereference.

Reviewed By: sbuggay

Differential Revision: D108428720

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@zeyap has exported this pull request. If you are a Meta employee, you can view the originating Diff in D108428720.

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This pull request has been merged in dd5c383.