Fix "Silent" summary for ringtone preference

fixes #3962
Closes #4441

//FREEBIE

res/values/strings.xml

@@ -917,6 +917,7 @@
     <string name="preferences__pref_led_blink_custom_pattern_set">Custom LED blink pattern set!</string>
     <string name="preferences__sound">Sound</string>
     <string name="preferences__change_notification_sound">Change notification sound</string>
+    <string name="preferences__silent">Silent</string>
     <string name="preferences__inthread_notifications">In-thread notifications</string>
     <string name="preferences__play_inthread_notifications">Play notification sound when viewing an active conversation</string>
     <string name="preferences__repeat_alerts">Repeat alerts</string>

src/org/thoughtcrime/securesms/preferences/NotificationsPreferenceFragment.java

@@ -59,7 +59,7 @@ public class NotificationsPreferenceFragment extends ListSummaryPreferenceFragme
       String value = (String) newValue;
 
       if (TextUtils.isEmpty(value)) {
-        preference.setSummary(R.string.preferences__default);
+        preference.setSummary(R.string.preferences__silent);
       } else {
         Ringtone tone = RingtoneManager.getRingtone(getActivity(), Uri.parse(value));
         if (tone != null) {