[Regional Preference] Append unicode tags to new system language.

-When the user adds a new system language, this new language can not
 have the current unicode tags. Hence, it shall append current unicode tags to the new system language.

Bug: b/268025290
Test: Manual test passed.
Test: make RunSettingsRoboTests ROBOTEST_FILTER=LocaleListEditorTest
Change-Id: I76671908ad01dcb8b04714aa3ca44d3614a0428b

tests/robotests/src/com/android/settings/localepicker/LocaleListEditorTest.java

@@ -28,6 +28,7 @@ import android.widget.TextView;
 import androidx.appcompat.app.AlertDialog;
 import androidx.fragment.app.FragmentActivity;
 
+import com.android.internal.app.LocaleStore;
 import com.android.settings.R;
 import com.android.settings.testutils.FakeFeatureFactory;
 import com.android.settings.testutils.shadow.ShadowAlertDialogCompat;
@@ -44,6 +45,8 @@ import org.robolectric.RuntimeEnvironment;
 import org.robolectric.annotation.Config;
 import org.robolectric.util.ReflectionHelpers;
 
+import java.util.Locale;
+
 @RunWith(RobolectricTestRunner.class)
 @Config(shadows = ShadowAlertDialogCompat.class)
 public class LocaleListEditorTest {
@@ -160,4 +163,15 @@ public class LocaleListEditorTest {
 
         assertThat(shadowDialog.getMessage()).isNull();
     }
+
+    @Test
+    public void mayAppendUnicodeTags_appendUnicodeTags_success() {
+        LocaleStore.LocaleInfo localeInfo = LocaleStore.fromLocale(Locale.forLanguageTag("en-US"));
+
+        LocaleStore.LocaleInfo result =
+                LocaleListEditor.mayAppendUnicodeTags(localeInfo, "und-u-fw-wed-mu-celsius");
+
+        assertThat(result.getLocale().getUnicodeLocaleType("fw")).isEqualTo("wed");
+        assertThat(result.getLocale().getUnicodeLocaleType("mu")).isEqualTo("celsius");
+    }
 }