Fix accessibility issues in Private DNS Settings

- Keep the Save button enabled at all times

- Show error in the Hostname view to remind the user
  - "The field is required" error
  - "The hostname you typed isn't valid" error

Bug: 386323822
Flag: EXEMPT bugfix
Test: Manual testing
atest -c PrivateDnsModeDialogPreferenceTest \
         PrivateDnsPreferenceControllerTest

Change-Id: I63973bd5001b838d7f27827e6a6d4ac96ac78ca9

src/com/android/settings/network/PrivateDnsPreferenceController.java

@@ -135,7 +135,7 @@ public class PrivateDnsPreferenceController extends BasePreferenceController
                                 com.android.settingslib.R.string.private_dns_mode_opportunistic);
             case PRIVATE_DNS_MODE_PROVIDER_HOSTNAME:
                 return dnsesResolved
-                        ? PrivateDnsModeDialogPreference.getHostnameFromSettings(cr)
+                        ? ConnectivitySettingsManager.getPrivateDnsHostname(mContext)
                         : res.getString(
                                 com.android.settingslib.R.string.private_dns_mode_provider_failure);
         }