google_login: use a TALES expression for secret key field

This way, guarded_getattr is used and accessor permission is applied.

bt5/erp5_oauth_google_login/SkinTemplateItem/portal_skins/erp5_oauth_google_login/SystemPreference_viewGoogleOAuthPreference/my_preferred_google_secret_key.xml

@@ -10,6 +10,7 @@
             <key> <string>delegated_list</string> </key>
             <value>
               <list>
+                <string>default</string>
                 <string>title</string>
               </list>
             </value>
@@ -48,6 +49,12 @@
             <key> <string>tales</string> </key>
             <value>
               <dictionary>
+                <item>
+                    <key> <string>default</string> </key>
+                    <value>
+                      <persistent> <string encoding="base64">AAAAAAAAAAI=</string> </persistent>
+                    </value>
+                </item>
                 <item>
                     <key> <string>field_id</string> </key>
                     <value> <string></string> </value>
@@ -63,6 +70,10 @@
             <key> <string>values</string> </key>
             <value>
               <dictionary>
+                <item>
+                    <key> <string>default</string> </key>
+                    <value> <string></string> </value>
+                </item>
                 <item>
                     <key> <string>field_id</string> </key>
                     <value> <string>my_string_field</string> </value>
@@ -81,4 +92,17 @@
       </dictionary>
     </pickle>
   </record>
+  <record id="2" aka="AAAAAAAAAAI=">
+    <pickle>
+      <global name="TALESMethod" module="Products.Formulator.TALESField"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_text</string> </key>
+            <value> <string>here/getPreferredGoogleSecretKey</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
 </ZopeData>