For future learners:
I decided to search for a different image classification android app (different to the one provided by the TF D&D specialisation) on github which worked well with my model and eventually I found it: “GitHub - IJ-Apps/Image-Classification-App-with-Teachable-Machine: Android app that uses a TensorFlow Lite model for image classification of common objects, trained through Google's Teachable Machine.”
In this image classification android app, if you simply change the model given (“model.tflite”) with your own (“converted_model.tflite”), and change the labels given {“Banana”, “Orange”, “Pen”, “Sticky Notes”} with your own {“Rock”, “Paper”, “Scissors”}, the app works well. i.e.: the accuracy of predictions is reasonable.