title: Select first n rows in contentProvider
body: <p>I've this select:</p>
  
  <pre><code> final Cursor cursorConversations = getContentResolver().query(
          Uri.parse("content://gmail-ls/conversations/" + Uri.encode(mailAddress)),
          null, null, null, BaseColumns._ID + " DESC");
  
   ContentQueryMap mQueryMap 
          = new ContentQueryMap(cursorConversations, BaseColumns._ID, true, null);
  </code></pre>
  
  <p>With ContentQueyMap I can cache Cursor data and iterate in it also with the Cursor closed (i need it to boost performance).</p>
  
  <p>Now, I want that the select of the Corsor only retrieve the first fifty rows. The solution of looping for 50 times in <code>mQueryMap.getRows().entrySet()</code> is not right: I don't want that mQueryMap gets all the rows of the Cursor, but only the first fifty.</p>
  
  <p>Any idea? Does exist a where clause to get only first n rows?</p>