iOS: Deprecate FlutterEngine.isGpuDisabled (#191393)

`FlutterEngine.isGpuDisabled` is currently exposed as a public
read/write property on `FlutterEngine` in the iOS framework API
(`FlutterEngine.h`).

This property is used internally by the engine to track whether the app
currently has access to the GPU. We update it on lifecycle transitions
between foreground and background, as well as on startup and when the
view is mounted/unmounted from the view hierarchy.

This state management was originally added to prevent GPU work from
running on the IO thread pool when the application loses access to the
GPU. While we do need to expose a read/write property for tests, there
was no clear need for it to be exposed as public API, and should have
been exposed on `FlutterEngine_Internal.h`.

Issue: https://github.com/flutter/flutter/issues/191347

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