Add minimal Android WebView app and GitHub Actions to build debug APK

[lan2015se-collab]

Motivation

• Provide a minimal Android WebView application that embeds https://iwd.illusd.com/ as an installable APK.
• Add CI to produce a debug APK artifact for every push and pull request.
• Exclude build outputs and generated artifacts from VCS with a .gitignore.

Description

• Added an Android application including MainActivity.java, AndroidManifest.xml, resource files (strings.xml, colors.xml, styles.xml), and the app Gradle module (app/build.gradle).
• Added top-level Gradle configuration files (build.gradle, settings.gradle) and project README.md with local build instructions.
• Added .github/workflows/build-apk.yml which runs gradle assembleDebug on push, pull_request, and workflow_dispatch and uploads the iwd-debug-apk artifact containing app-debug.apk.
• Added .gitignore entries to ignore Gradle and build artifacts and binary APK/AAB files.

Testing

• No automated tests were executed as part of this PR.
• A GitHub Actions workflow Build APK was added and is configured to run gradle assembleDebug and upload the iwd-debug-apk artifact on push, pull_request, or manual dispatch.

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Codex Task