Keep valid SKUs in memory

[huttj]

I get an error when trying to purchase a product after calling getSubscriptions(subSkus) and getProducts(productSkus), like so:

const subscriptions = await RNIap.getSubscriptions(subscriptionIds);
const products = await RNIap.getProducts(productIds);

This seems to be because both call RNIapModule.getItemsByType(), which replaces the internal SKUs cache. The only way to get it to work without making this change is to call the getProducts() or getSubscriptions() before trying to purchase.

[hyochan]

Great work! Much better to handle the side effect!