With inspiration from @Andy Developer, I could display the map after my app was uploaded to Google Play. These were the steps:
- Upload your app to Google Play (signed APK file)
- After the app has been submitted and approved, select your app from Google Play Console
- Go to Development Tools -> Release management -> App signing
- Copy the first SHA-1 certificate which Google Play has issued after uploading the app.
- Go to Google Console and select your project.
- Select your API-key, restrict your key, and paste the SHA-1 after package name.
- Press Save, wait a couple of mins. and your app should show Google Maps.