Refactor canMakePayments to BillingWrapper

[beylmk]

Currently, canMakePayments lives in Purchases. We will need to leave an API to call canMakePayments there, but should refactor the actual code into BillingWrapper to respect our blackboxing of store-specific code. The same will be needed for isBillingSupported and isFeatureSupported. This might require a Store parameter to be passed to the method, since it's static.

Additionally, we should add comments and/or perhaps errors to clarify that this method currently only exists for the Play store (there is no known implementation for Amazon at this point).

[vegaro]

Just adding some more context here, because we were talking about it in grooming. We think for Amazon, returning true would be a good compromise. Apart from that, when we move the call to BillingWrapper, the call should be static, since it doesn't require an instance of BillingWrapper. Maybe it should live in a new class that's a static class, since BillingWrapper is not.