title: How to check if the user has bought a subscription
body: <p>in my android app i implemented the in App-Billing, where the user can buy a subscription in the app.
  when the user launch the app, i want to check if he has any subscription at all, to decide which content should i show him.
  i'm using the latest in-app-billing library <code>implementation 'com.android.billingclient:billing:1.1'</code> 
  thank you in advance for the help</p>