[usecase-webapi][XWALK-893] Add a usecase for IAP

- Add a usecase for IAP to query product details and buy gas product
- Failure analysis: "In App Purchase" feature does not allowed in China

Impacted tests(approved): new 1, update 0, delete 0
Unit test platform: [Android]
Unit test result summary: pass 0, fail 1, block 0
Can you point me why use this magic string?