Added InAppBilling.VerifyPreviousPurchaseAsync()

[rihadavid]

So I was thinking, what should I do if user purchases the product in the store but the verification on our server fails. I was looking for a way to call the verification on this purchase again, but did not find a solution.

Calling InAppBillingImplementation.GetPurchasesAsync() with verification will try to verify all the previous purchases, which is not what I want - it will fail on the server because it had been verified before and also the server request will be pointless for those already verified purchases. And if user just tries to purchase the product again, it will fail with PurchaseError.AlreadyOwned and the verification will not be performed, right?

I've been thinking about many ways how this could be implemented, but I think this one is the easiest. Tell me what you think, it's my first PR here.

Changes Proposed in this pull request:

• added BaseInAppBilling.VerifyPreviousPurchaseAsync() method to support verifying a single previous purchase, internally using GetPurchasesAsync() method with verification limited to the specified product
• changed InAppBillingImplementation.GetPurchasesAsync() methods to protected to support the additional parameter and moved the original public method to BaseInAppBilling

[InquisitorJax]

@jamesmontemagno This would definitely be useful