Have you checked out Burp proxy?
I always use Brup for any Android app, desktop app, website... and you don't even have to use ProxyDroid.
Connect you mobile phone and your laptop to the same LAN and configure the Burp proxy to start listening at your private ip. Then in android wifi options you can configure the proxy settings ip and port:
After that configure Brup proxy options under the proxy tab to stop all requests and responses. After that you don't need to know the port that you app is using, all traffic from you smartphone will be shown in Brup.
Apart from that Burp have lots of features and plugins for fuzzing and other stuff, I don't know what are you trying to accomplish here but anyway this way should work for you!