performPurchase
Performs an in-app purchase for the specified package.
If a purchase is successful, syncPurchases will automatically be called by RevenueCat to update our database. However, if you are using Amazon's store, you must call syncAmazonPurchase in your code.
Return
A PurchaseLogicResult object containing the outcome of the purchase operation.
Parameters
activity
The current Android Activity triggering the purchase.
rcPackage
The package representing the in-app product that the user intends to purchase.