Fixed accessibility issues in VPN Settings

- Show "(required)" and errors in required fields to alert users

- Show "(optional)" below each optional field

Bug: 386025633
Flag: EXEMPT bugfix
Test: Manual testing
  atest WifiConfigController2Test
Change-Id: Iefbd68e6658af7b073db219b3e04e94805092759

src/com/android/settings/vpn2/ConfigDialogFragment.java

@@ -124,6 +124,7 @@ public class ConfigDialogFragment extends InstrumentedDialogFragment implements
         VpnProfile profile = dialog.getProfile();
 
         if (button == DialogInterface.BUTTON_POSITIVE) {
+            if (!dialog.validate()) return;
             // Possibly throw up a dialog to explain lockdown VPN.
             final boolean shouldLockdown = dialog.isVpnAlwaysOn();
             final boolean shouldConnect = shouldLockdown || !dialog.isEditing();