[in_app_purchase] remove AndroidX constraint

[cyanglaz]

Description

After 0.2.2, we introduced a constraint that the app uses the plugin has to be migrated to androidx.
This patch removes the constraint.

Related Issues

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
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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).
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[mklim]

LGTM