Add help info at the bottom of the dialog

It contains a link to help center

Bug: 68030013
Test: RunSettingsRoboTest
Change-Id: I79260eff35e604fa97cf21c62f58c02f3bbe5cfb

tests/robotests/src/com/android/settings/network/PrivateDnsModeDialogFragmentTest.java

@@ -50,7 +50,6 @@ public class PrivateDnsModeDialogFragmentTest {
     private PrivateDnsModeDialogFragment mFragment;
     private Button mSaveButton;
 
-
     @Before
     public void setUp() {
         MockitoAnnotations.initMocks(this);