title: Google Play library 3.0 check if user used introductory price
body: <p>I am using Google Play Billing library 3.0.3 with Java.
  I have an introductory price on my subscriptions.</p>
  <p>When the user already have used the introductory price, the getIntroductoryPrice() function from Google billing library still returns the introductory price.</p>
  <p>How can I check if the user already used the introductory price for not showing it again to the user?</p>
  <p>I want to show the introductory price just the first time, when the user have not yet used it.</p>