Hmm. Re-scaling a Canvas with Sprites for different screen sizes is a bit of a nightmare (but possible...your code works on your device using scaling 
)
You either need several Android devices to test your app's behavior or use a more sophisticated emulator than the standard emulator App Inventor 2 provides ( Android's basic emulator).
- you might use a GenyMotion emulator that can simulate various devices and Screen sizes instead of the MIT provided emulator.
Genymotion Android Emulator | Cloud-based Android virtual ...
If you decide to use GenyMotion (there is a 'free' version), you need to use a special Companion Emulator setup Genymotion when you use it with App Inventor.