title: Fragment back stack does not work when extending AppCompatActivity
body: <p>I'm using the new <code>AppCompatActivity</code> introduced in the <code>AppCompat</code> library version 22.1.</p>
  
  <p>When I extend this Activity, the hardware back button no longer pops the back stack of my Fragments, it closes the Activity instead.</p>
  
  <p>Here is how I'm changing fragments in my activity: </p>
  
  <pre><code>public void changeFragment(Fragment f) {
      FragmentTransaction ft = getFragmentManager().beginTransaction();
      ft.replace(R.id.fragment_holder, f);
      ft.addToBackStack(null);
      ft.commit();
  }
  </code></pre>
  
  <p>If I change <code>MainActivity extends AppCompatActivity</code> to <code>MainActivity extends Activity</code> the problem goes away and I am able to go backwards through my fragments.</p>
  
  <p>Changing calls to <code>getFragmentManager()</code> to <code>getSupportFragmentManager()</code> results in devices running Android &lt; 5.0 losing the Material theme, which was the main reason for implementing <code>AppCompatActivity</code> in the first place.</p>
  
  <p>The style referenced in my manifest <code>&lt;application android:theme="@style/AppTheme"&gt;</code></p>
  
  <pre><code>&lt;style name="AppTheme" parent="Theme.AppCompat.Light"&gt;
      &lt;item name="colorPrimary"&gt;@color/primary_material_light&lt;/item&gt;
      &lt;item name="colorPrimaryDark"&gt;@color/primary_dark_material_light&lt;/item&gt;
      &lt;item name="colorAccent"&gt;@color/accent_material_light&lt;/item&gt;
  &lt;/style&gt;
  </code></pre>