lib: Fix saving of remote titles

We were using the key 'title' in some places, but 'xa.title'
elsewhere. Make it 'xa.title' everywhere. This bug was found
by the new unit tests.

lib/flatpak-remote.c

@@ -579,7 +579,7 @@ flatpak_remote_commit (FlatpakRemote   *self,
     g_key_file_set_string (config, group, "url", priv->local_url);
 
   if (priv->local_title_set)
-    g_key_file_set_string (config, group, "title", priv->local_title);
+    g_key_file_set_string (config, group, "xa.title", priv->local_title);
 
   if (priv->local_gpg_verify_set)
     {