[Android] Don't call SetOffer for one-time purchases

[overzealus]

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Bug Information

Recently Google Play started to send messages of incorrect usage of billing library.
In this email they say that if you call setOfferToken with "" value, you should remove this calls.
In https://developer.android.com/google/play/billing/integrate#launch Google notice that
For One-time product, "setOfferToken" method shouldn't be called.
For subscriptions, to get an offer token, call ProductDetails.subscriptionOfferDetails() for a list of offers that are available to the user

Version Number of Plugin: 7.1.0
Device Tested On: Android with Google Play
Simulator Tested On: no
Version of VS: 17.9.3
Version of Xamarin: 17.9.0.138
Versions of other things you are using:

Steps to reproduce the Behavior

Expected Behavior

Don't call setOfferToken if offer Token is not provided by calling method.

Actual Behavior

setOfferToken is called -> Google sends warnings

Code snippet

Screenshotst

[overzealus]

Can you tell when updated nuget package would be available?