fix: Prevent multiple initializations of internal resources when RtcEngine.initialize is called simultaneously

[littleGnAl]

A case like:

for (int i = 0; i < 5; ++i) {
  rtcEngine.initialize(RtcEngineContext(
    appId: engineAppId,
    areaCode: AreaCode.areaCodeGlob.value(),
    logConfig: LogConfig(filePath: logPath),
  ));
}

This causes irisMethodChannel.initilize(args), and _initializeInternal(context) called multiple times, which causes unexpected behaviors.

iris_method_channel fix: AgoraIO-Extensions/iris_method_channel_flutter#98