[WIP] Migrate Flutter Android Test apps to AGP 9 - #185118

Upgraded Flutter Android test apps to AGP 9. Had to migrate any apps applying KGP to built-in Kotlin. Also convenient to update templates in this PR.

Fixes #179644
Related to: #185221

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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