Add help url placeholders for new pages

Bug: 38412798
Test: visual
Change-Id: I4ede523cb13790ea05309279f80a778b2086f850

src/com/android/settings/ScreenPinningSettings.java

@@ -74,6 +74,11 @@ public class ScreenPinningSettings extends SettingsPreferenceFragment
         mSwitchBar.setChecked(isLockToAppEnabled(getActivity()));
     }
 
+    @Override
+    protected int getHelpResource() {
+        return R.string.help_url_screen_pinning;
+    }
+
     @Override
     public void onViewCreated(View view, Bundle savedInstanceState) {
         super.onViewCreated(view, savedInstanceState);