So, I have included paypal-android-sdk: 2.15.3 in my application and it works as expected in Sandbox. Now I want to go to production.
I switched the environment to PayPalConfiguration.ENVIRONMENT_PRODUCTION AND customer ID to production customer ID. What happens is that firstly, I cannot log in using my real PayPal account, and secondly, I see the blue Sandbox button, as shown below:

my code is:
PayPalConfiguration config = new PayPalConfiguration()
.environment(CustomBuildConfig.PAY_PAL_CONFIG_ENVIRONMENT)
.clientId(CustomBuildConfig.PAY_PAL_CONFIG_CLIENT_ID);
PayPalPayment thingToBuy = new PayPalPayment(new BigDecimal(value), currency, item,
PayPalPayment.PAYMENT_INTENT_SALE);
Intent intent = new Intent(getContext(), PaymentActivity.class);
intent.putExtra(PayPalService.EXTRA_PAYPAL_CONFIGURATION, config);
intent.putExtra(PaymentActivity.EXTRA_PAYMENT, thingToBuy);
startActivityForResult(intent, REQUEST_CODE_PAYPAL_PAYMENT);
Where:
PAY_PAL_CONFIG_ENVIRONMENT = PayPalConfiguration.ENVIRONMENT_PRODUCTION; PAY_PAL_CONFIG_CLIENT_ID = "production customer identifier";
Edit: The same work product id is already in use and works for the iOS client.