title: ActionBar 'up' button destroys parent activity, 'back' does not
body: <p>I have a relatively simple Android app with one <code>Activity</code> showing a list of items and another showing details of a selected item.  I start the list activity, which is my topmost activity (using <code>FLAG_ACTIVITY_CLEAR_TOP</code> to clear the login activity from which this is called) with:</p>
  
  <pre><code>Intent intent = new Intent(this, ListInstancesActivity.class);
  intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
  startActivity(intent);
  finish();
  </code></pre>
  
  <p>and from within that activity I act on an item being selected with:</p>
  
  <pre><code>Intent detailIntent = new Intent(this, ShowInstanceActivity.class);
  detailIntent.putExtra(ShowInstanceFragment.ARG_ITEM_ID, id);
  startActivity(detailIntent);
  </code></pre>
  
  <p>All works fine, and if I use the softkey 'back' button then I return to the <code>ListInstancesActivity</code> as I would expect.  However, if instead I press the back/up button on the action bar then it destroys and recreates the <code>ListInstancesActivity</code>.  This is bad, as it is relatively computationally expensive to do so.</p>
  
  <p>How can I make the action bar behave in the same way as the softkey and just return to the previous activity rather than destroying it.</p>
  
  <p>It should be noted that I'm using the support library version of the actionbar.</p>
  
  <p>The relevant parts of my <code>AndroidManifest.xml</code> are</p>
  
  <pre><code>&lt;activity
    android:name=".agenda.ListInstancesActivity"
    android:label="@string/list_instances_activity_title"&gt;
  &lt;/activity&gt;
  &lt;activity
    android:name=".agenda.ShowInstanceActivity"
    android:label="@string/show_instance_activity_title"
    android:parentActivityName=".agenda.ListInstancesActivity"&gt;
  &lt;/activity&gt;
  </code></pre>