Add a method to check if service is initialized and usable

[coderGtm]

I have a use case where I don't want to initialize the service in onCreate() in order to save memory. So I initialize it just before purchase. But if I initialize it everytime on a purchase, it will call addPurchaseListenerer all those times. So it here a way to know that the service is initialized and usabe (if it is initialized long back, say a day, then the session might get expired....just my guess).

So a function like iapConnector.isUsable() will be awesome!