updates the billing client version to use for unity IAP compatibility…

[aboedo]

I was getting the following error while trying a Unity-IAP compatible build.
It looks like Unity IAP updated their billing client version. The update was done in [3.1.0] - 2021-04-15, according to this changelog

Duplicate class com.android.billingclient.BuildConfig found in modules 
billing-3.0.3-runtime.jar (:billing-3.0.3:) and 
com.android.billingclient.billing-3.0.2-runtime.jar (:com.android.billingclient.billing-3.0.2:)

[aboedo]

@vegaro can I get a review for this one?
I'd love to do a release for this one, then propagate to the hybrids so we can release Unity today

[vegaro]

Weird you were getting that error because the unity iap version of purchases-android-unityiap version shouldn't be bringing the billing client automatically 🤔

[vegaro]

#566 fixes the unintentional exposure of the billing client to consumers