title: best way to access IabHelper from my customs list view adapter
body: <p>I have a IabHelper in my MainActivity which I'm using to for initial setup and for querying my in-app products.</p>
  
  <p>Then I'm creating a listview with all the products, every item in my list view has a "BUY" button which should trigger a purchase for this item.</p>
  
  <p>I'm using a custom adapter and I added a onClickListener in my getView function.</p>
  
  <p>Now my question is what is the best way to use the IabHelper member that is declared in my MainActivity to start a purchase from my CustomAdapter class ?</p>
  
  <ul>
  <li>singleton</li>
  <li>declare the IabHelper in my Application class</li>
  <li>use a static variable (but then I should do it to all the IabListeners?)</li>
  <li>any other idea...?</li>
  </ul>