[in_app_purchase_android] Bump gradle plugin to 9.1.1

[camsim99]

Bumps com.android.tools.build:gradle from 8.13.1 to 9.1.1 in packages/in_app_purchase/in_app_purchase_android.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

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[gemini-code-assist]

Code Review

This pull request updates the in_app_purchase_android package to version 0.4.1, which includes bumping the com.android.tools.build:gradle dependency from 8.13.1 to 9.1.1. I have no feedback to provide.