Settings: Fix invalid private DNS help text if URI is missing

AOSP does not have help URIs configured, so there is no relevant content
to show in the private DNS dialog's help text section, and the preference
controller never sets the content of the help text view. This results in
a random irrelevant message related to Bluetooth audio being shown in
the dialog: "Gray-out means not supported by phone or headset"

This is caused by the private DNS dialog sharing the
preference_widget_dialog_summary snippet with Bluetooth A2DP codec
settings. To fix this issue, we can hide the help text view entirely if
there is no help URI defined.

Change-Id: Ie74a1be347351a6db68412321d0c68f3138fae6b
Signed-off-by: Joey Huab <joey@evolution-x.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>

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

@@ -220,7 +220,7 @@ public class PrivateDnsModeDialogPreference extends CustomDialogPreferenceCompat
             helpTextView.setText(AnnotationSpan.linkify(
                     context.getText(R.string.private_dns_help_message), linkInfo));
         } else {
-            helpTextView.setText("");
+            helpTextView.setVisibility(View.GONE);
         }
 
         updateDialogInfo();