How to correctly import BillingResponse with newest version of billing client (billing:2.0.1)
Check https://developer.android.com/google/play/billing/billing_library_releases_notes
BillingResult return values
APIs that previously returned a BillingResponse integer value now return a BillingResult object. BillingResult contains the BillingResponse integer as well as a debug string that you can use to diagnose errors. The debug string uses an en-US locale and is not meant to be shown to end users.