v13.8.0

Upgrade to Google Play Billing library 5.2.1

Adds access to offer and base plan identifiers.

Handle validator answer with code VALIDATOR_SUBSCRIPTION_EXPIRED

For backward compatibility, the validator also support responses with a 6778003
error code (expired) when the validated transaction is expired.

Fix: AppStore adapter should only return a localReceipt on iOS

A dummy appstore receipt was listed on other platforms, this is fixed.

Prevent various issues

Prevent double calls to approved callbacks

Make sure .approved() is only called once during a small time frame.

Skip quick successive calls to store.update()

The update will be performed only if store.update() or store.initialize()
was called less than store.minTimeBetweenUpdates milliseconds.

This make it safer to always call store.update() when entering the app's
Store screen.

Block double callback registrations

Throw an error when attempting the re-register an existing callback for a given
event handler. This is indicative of initialization code being run more than
once.