isActive method
Return true
if there is at least one purchase that unlocked this
entitlement. It's possible the user didn't purchase a SKU on user's
current device platform but they did make a transaction on a different
device platform. To enable the entitlement under this context,
activePurchases will have at least one item to activate the entitlement.
Implementation
bool isActive() {
return purchasedSKUs.isNotEmpty || activePurchases.isNotEmpty;
}