[in_app_purchase] Add support to cross-grade a subscription on Android

[lexor90]

This change introduces basic support to replace an old subscription on
Android. On iOS passing the added parameter will have no effect.

Please note that the current implementation is basic, in that it misses
a way to contextually change the proration mode.

It also uses setOldSku(string) rather than setOldSku(string, string)
which also requires the old purchaseToken to be provided.

Ref: https://developer.android.com/reference/com/android/billingclient/api/BillingFlowParams.Builder#setoldsku

Description

Hi, I’ve done a basic implementation to support Android subscription cross grades.

Note that this is a basic implementation in that it doesn’t support customizing the proration mode while changing the subscription. It also uses the deprecated method setOldSku(string). In order to use the new setOldSku(string, string) method we should add another parameter to accept the "purchaseToken", which seemed too invasive with regards to the iOS counterpart which doesn’t need that information as cross-grades are managed by the storefront through the concept of groups. I'm open to suggestions to add support for custom proration mode and to use the new suggested setSku(string, string) method.

I preferred the deprecated method in order to keep the change as simple as possible while achieving what most apps need. On iOS passing the added oldProduct parameter will have no effect.

Related Issues

• [in_app_purchase] Support subscription upgrade/downgrade flutter#32395
• How to buy subscription upgrade flutter#51535

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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[cacianokroth]

👁️

[Ainiuso]

👁️

[cacianokroth]

Any predictions or answers for this PR?

[TrueNight]

I have already checked this functionality, it works like a charm!

[lexor90]

Closing in favour of #2822 that got merged.