Add Android build action

[GSala]

Description

Add build-android, a Node.js 24 action that runs Gradle for a selected module and one or more variants, then exposes APK/AAB and merged manifest paths for subsequent workflow steps.

• Support APK and app bundle builds, nested modules, flavored variants, split APKs, extra Gradle arguments, and project paths containing spaces.
• Scope discovery to the requested module and variants, retain valid up-to-date outputs, and return deduplicated, sorted absolute paths.
• Fail when Gradle fails or no matching artifact exists; warn when merged manifests are unavailable.
• Include the bundled runtime, regression tests, and usage documentation. Register the action in the rebuild workflow, Dependabot, and repository README.

Motivation and Context

Provide a shared Android build step with predictable outputs that can be passed to existing upload and distribution actions.

Validation

• Fresh dependency installation on Node.js 24; npm run all passed formatting, lint, TypeScript checks, packaging, and all 25 tests.
• Tests cover filesystem discovery, wrapper execution, GitHub output exports, and failure handling.
• Rebuilt distribution files match the checked-in bundle.
• Tests use temporary Gradle stubs. A smoke build against a real Android project remains outstanding.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)