Correct default value of 'pref_key_ringtone'.

Fixes #2793
Closes #2831

// FREEBIE

src/org/thoughtcrime/securesms/util/TextSecurePreferences.java

@@ -3,10 +3,9 @@ package org.thoughtcrime.securesms.util;
 import android.content.Context;
 import android.os.Build;
 import android.preference.PreferenceManager;
+import android.provider.Settings;
 import android.util.Log;
 
-import org.thoughtcrime.securesms.R;
-
 import java.io.IOException;
 
 public class TextSecurePreferences {
@@ -378,7 +377,7 @@ public class TextSecurePreferences {
   }
 
   public static String getNotificationRingtone(Context context) {
-    return getStringPreference(context, RINGTONE_PREF, null);
+    return getStringPreference(context, RINGTONE_PREF, Settings.System.DEFAULT_NOTIFICATION_URI.toString());
   }
 
   public static boolean isNotificationVibrateEnabled(Context context) {