Android/WinRT: Expose purchaseToken (receipt) for secure in app purchase verification

The purchase token (or receipt on WinRT) is useful to verify a purchase with the Google/Microsoft servers. At the moment this token is not exposed in the API so in app purchases made with Qt Purchase are easy to hack by faking the receipt.

On iOS this is not necessary because the receipt is stored globally and is not passed as a part of the purchasing transaction.