Fix Book Creator settings (switch to `JSON.stringify`)

Part of the removal of jquery.json, partially done in patches:
* I354375a9a32a5ce379aca19b2d7c4c4114748c72
* Ib37dc3efc09b61264e462404b0befae43d93a734

Bug: 72003
Change-Id: I0532514144a3eaff13e2498c739de6bbfb89f8a4