title: Activity.findViewById() returns null in Espresso test
body: <p>In an Espresso test, I want to get reference to my <code>ViewPager</code> so I can attach an <code>IdlingResource</code> to it. But <code>findViewById</code> returns <code>null</code>, even though I confirm just before that the <code>ViewPager</code> is displayed.</p>
  
  <pre><code>// Passes
  onView(withId(R.id.viewPager)).check(matches(isDisplayed()));
  Activity activity = activityTestRule.getActivity();
  assertNotNull(activity);
  // Fails
  assertNotNull(activity.findViewById(R.id.viewPager));
  </code></pre>
  
  <p>Any ideas?</p>