[IAP] query and buy products

Includes following:

1. Two features:
1.1 Query Details of Products
1.2 Buy Products

2. IABHelper Class
2.1 Based on Android IAB official example
2.2 Added license information
2.3 Added toJson() to SkuDetails
2.4 Added getAllProducts() to Inventory

3. IInAppBillingService.aidl

Following has details of how to use this AIDL to generate interface class to
Android's In App Billing, and how to build the IAP Extension:
https://github.com/crosswalk-project/crosswalk-android-extensions/wiki/Build-IAP-Extension