fix: Fix MediaEngineImpl.unregisterAudioFrameObserver not unregister …

…eventhandler internally (#1495)

Wrong call `irisMethodChannel.registerEventHandler` inside
`MediaEngineImpl.unregisterAudioFrameObserver`, it should be
`irisMethodChannel.unregisterEventHandler`

lib/src/impl/agora_media_engine_impl_override.dart

@@ -70,7 +70,7 @@ class MediaEngineImpl extends media_engine_impl_binding.MediaEngineImpl
     final eventHandlerWrapper = AudioFrameObserverWrapper(observer);
     final param = createParams({});
 
-    await irisMethodChannel.registerEventHandler(
+    await irisMethodChannel.unregisterEventHandler(
         ScopedEvent(
             scopedKey: _mediaEngineScopedKey,
             registerName: 'MediaEngine_registerAudioFrameObserver',