api: add new api to register insert/remove callback

api: add new api to register insert/remove callback

- Previously, storage event register apis get the storage ID
  by the input parameter. Storage IDs are created when
  storages are connected, and storage IDs are changed
  even if same storage is re-connected. Thus Previous apis
  cannot support "insert" event.

- Now, new apis are added. They get storage type to register
  events. Thus they can support "insert" event too.

   = int storage_set_changed_cb(storage_type_e type, storage_changed_cb callback, void *user_data);
   = int storage_unset_changed_cb(storage_type_e type, storage_changed_cb callback);

Change-Id: Icbc6513f2777f8df760365c5c65512714b295590
Signed-off-by: taeyoung <ty317.kim@samsung.com>