[codex] Add Android Play internal deploy

[jjoonleo]

Summary

• Add a manual Android Play Internal Testing deploy workflow for signed draft .aab uploads.
• Decode Firebase config and the upload keystore from protected release environment secrets.
• Document the required GitHub environment secrets, variable, and CI signing setup.

Validation

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/android-play-internal.yml"); puts "yaml ok"'
• git diff --cached --check

Notes

• The workflow must be dispatched from main.
• The release environment should have required reviewers configured in GitHub settings.

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