Improve Android product & inventory retrieval

[ened]

• Only submit requests to the Billing engine when the initialization is done
• Re-Create the billing manager for each interaction to avoid overlaps
• Add a new method to retrieve the list of SKUs a user has currently purchased (Android only, for the moment)

This should also fix https://github.com/JackAppDev/flutter_iap/issues/18

Currently, the fetchInventory does not retrieve active subscriptions, and does not work on iOS. I can work on this in about 2 months.

[JackAppDev]

Could you include an updated readme and example?

[ened]

@JackAppDev ok, take a look