[camera] Guard CameraController value updates after dispose

[motucraft]

Guards the value assignments in _initializeWithDescription so they no longer run after dispose(), fixing the "A CameraController was used after being disposed." crash.

These user-level steps, using the reproduction app in flutter/flutter#184959, trigger the crash:

1. Open camera A.
2. Close it before initialization finished (disposing the controller).
3. Open camera B.
4. Adjust an exposure/zoom slider.

When the controller is disposed while initialization is still in flight, the deferred value updates could call notifyListeners() on a disposed ChangeNotifier, throwing the exception above.
This adds if (!_isDisposed) guards (and awaits the initialization event only once) so those updates are skipped after dispose.
The guard lives in shared Dart code, so it also covers iOS and web.

Part of flutter/flutter#184959.

This PR addresses only the Dart-layer crash. As @mbcorona noted in flutter/flutter#184959 (comment), the ANR observed on Android has a separate native root cause (ImageReader not severing the stream on dispose, "BufferQueue has been abandoned"), which was routed to team-android. That native issue is not addressed here and should be handled separately in camera_android_camerax.

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