MDL-63303 message: fix get_conversations external func

lib/db/services.php

@@ -1086,6 +1086,7 @@ $functions = array(
         'description' => 'Retrieve a list of conversations for a user',
         'type' => 'read',
         'services' => array(MOODLE_OFFICIAL_MOBILE_SERVICE),
+        'ajax' => true
     ),
     'core_message_get_conversation' => array(
         'classname' => 'core_message_external',

message/externallib.php

@@ -1609,7 +1609,7 @@ class core_message_external extends external_api {
         return new external_single_structure(
             [
                 'conversations' => new external_multiple_structure(
-                    self::get_conversation_structure()
+                    self::get_conversation_structure(true)
                 )
             ]
         );