[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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[gemini-code-assist]

Code Review

This pull request prevents CameraController from updating its value after being disposed by adding _isDisposed checks in the device orientation listener, the camera error listener, and after awaiting initialization. It also includes corresponding unit tests and bumps the package version to 0.12.1. Feedback suggests cancelling and nullifying _deviceOrientationSubscription in the PlatformException catch block of _initializeWithDescription to prevent resource leaks and ensure successful retries.

[gemini-code-assist]

If _initializeWithDescription fails with a PlatformException, the _deviceOrientationSubscription that was created at the start of the method is left active and is never cancelled. This leads to a resource leak, and also prevents the subscription from being recreated if initialize() is retried (since _deviceOrientationSubscription remains non-null).

We should cancel the subscription and set it to null in the catch block to ensure proper cleanup and allow successful retries.

Suggested change

	} on PlatformException catch (e) {
	throw CameraException(e.code, e.message);
	} on PlatformException catch (e) {
	unawaited(_deviceOrientationSubscription?.cancel());
	_deviceOrientationSubscription = null;
	throw CameraException(e.code, e.message);

[motucraft]

/gemini

This is pre-existing behavior unrelated to the bug this PR fixes (a CameraController updating its value after being disposed).

The subscription is already cancelled in dispose(). It only lingers if the controller is never disposed, which is independent of whether initialization succeeds or fails. And since onDeviceOrientationChanged() is a global stream, reusing it on retry is intentional rather than a leak.

To keep this PR focused, I'd prefer to leave this out of scope, but I'm happy to address it in a separate PR if a maintainer thinks it's worth changing.