Show shadow when entity header starts scrolling.

- Add a controller to manage add/remove onScrollChangedListener to
  recyclerviews.
- When recyclerview on each screen is scrolled to top, set actionbar
  elevation to 0, otherwise set it to non-zero.
- When screen is moved to background, detach the listener.
- Use the controller in entity header.

Change-Id: Iecf194d885098c98c392810f62893ae9189f3936
Fix: 37670670
Test: make RunSettingsRoboTests

src/com/android/settings/notification/ChannelNotificationSettings.java

@@ -112,6 +112,7 @@ public class ChannelNotificationSettings extends NotificationSettingsBase {
         final Activity activity = getActivity();
         final Preference pref = EntityHeaderController
                 .newInstance(activity, this /* fragment */, null /* header */)
+                .setRecyclerView(getListView(), getLifecycle())
                 .setIcon(mAppRow.icon)
                 .setLabel(mChannel.getName())
                 .setSummary(mAppRow.label)