How to get a view from within Espresso to pass into an IdlingResource?

Figured it out. To get the view to pass into an idling resource, all you have to do is take the member variable of your ActivityTestRule

For example:

@Rule
public ActivityTestRule<MainActivity> activityTestRule = new ActivityTestRule<>(
        MainActivity.class);

and then just call getActivity().findViewById(R.id.viewId)

So the end result is:

activityTestRule.getActivity().findViewById(R.id.viewId);

The accepted answer works as long as a test is running in the same activity. However, if the test navigates to another activity activityTestRule.getActivity() will return the wrong activity (the first one). To address this, one can create a helper method returning an actual activity:

public Activity getCurrentActivity() {
    final Activity[] currentActivity = new Activity[1];
    InstrumentationRegistry.getInstrumentation().runOnMainSync(new Runnable() {
        @Override
        public void run() {
            Collection<Activity> allActivities = ActivityLifecycleMonitorRegistry.getInstance()
                    .getActivitiesInStage(Stage.RESUMED);
            if (!allActivities.isEmpty()) {
                currentActivity[0] = allActivities.iterator().next();
            }
        }
    });
    return currentActivity[0];
}

And then it could be used as the following:

Activity currentActivity = getCurrentActivity();
if (currentActivity != null) {
    currentActivity.findViewById(R.id.viewId);
}