title: findViewById() returns null for custom component in layout XML, not for other components
body: <p>I have a <code>res/layout/main.xml</code> including these elements and others:</p>
  
  <pre><code>&lt;some.package.MyCustomView android:id="@+id/foo" (some other params) /&gt;
  &lt;TextView android:id="@+id/boring" (some other params) /&gt;
  </code></pre>
  
  <p>In my Activity's onCreate, I do this:</p>
  
  <pre><code>setContentView(R.layout.main);
  TextView boring = (TextView) findViewById(R.id.boring);
  // ...find other elements...
  MyCustomView foo = (MyCustomView) findViewById(R.id.foo);
  if (foo == null) { Log.d(TAG, "epic fail"); }
  </code></pre>
  
  <p>The other elements are found successfully, but <code>foo</code> comes back null. MyCustomView has a constructor <code>MyCustomView(Context c, AttributeSet a)</code> and a <code>Log.d(...)</code> at the end of that constructor appears successfully in logcat just before the "epic fail".</p>
  
  <p>Why is <code>foo</code> null?</p>