[manager] OboeAudioDevice::stopCallback() should clean up stream.

This seems to fix the instancing issue at #199 (comment)

androidaudioplugin-manager/src/main/cpp/OboeAudioDeviceManager.cpp

@@ -143,6 +143,7 @@ void aap::OboeAudioDevice::stopCallback() {
     oboe::Result result = stream->stop();
     if (result != oboe::Result::OK)
         throw std::runtime_error(std::string{"Failed to stop Oboe stream: "} + oboe::convertToText(result));
+    stream = nullptr;
 }
 
 oboe::DataCallbackResult