Define "Add network" preference in XML.

Move the definition from Java to XML, so that SearchIndexProvider can
include it in the getXmlResourcesToIndex().
Also update getNonIndexableKeys() to return the corresponding key when
the preference is hidden.

If the Internet settings are restricted, "Add network" still appears on
search. When clicked, it opens the Internet settings page in restricted
mode, so that users can know that the settings are restricted.

Bug: 224421762
Fix: 224421762
Test: atest SettingsRoboTests:NetworkProviderSettingsTest
Change-Id: Ie9ea58070843a390844b461ce766f0ce29e64fab

src/com/android/settings/wifi/AddWifiNetworkPreference.java

@@ -20,10 +20,12 @@ import android.content.Context;
 import android.content.res.Resources;
 import android.graphics.drawable.Drawable;
 import android.os.UserManager;
+import android.util.AttributeSet;
 import android.util.Log;
 import android.widget.ImageButton;
 
 import androidx.annotation.DrawableRes;
+import androidx.annotation.Nullable;
 import androidx.preference.PreferenceViewHolder;
 
 import com.android.settings.R;
@@ -40,8 +42,11 @@ public class AddWifiNetworkPreference extends RestrictedPreference {
     private final Drawable mScanIconDrawable;
 
     public AddWifiNetworkPreference(Context context) {
-        super(context);
+        this(context, null);
+    }
 
+    public AddWifiNetworkPreference(Context context, @Nullable AttributeSet attrs) {
+        super(context, attrs);
         setLayoutResource(com.android.settingslib.R.layout.preference_access_point);
         setWidgetLayoutResource(R.layout.wifi_button_preference_widget);
         setIcon(R.drawable.ic_add_24dp);