Add Android precompile workflow

[dl-alexandre]

Summary

• add Android aarch64-linux-android and armv7a-linux-androideabi targets to the existing cc_precompiler configuration
• map Android precompile targets to the Android NDK clang toolchain while keeping the existing Linux/macOS/Windows targets unchanged by default
• add an Android precompile workflow that builds one Android target per matrix job and publishes matching release artifacts on tags

Fixes #165.

Validation

• mix format --check-formatted mix.exs
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/android-precompile.yml")'
• mix eval --no-compile --no-deps-check 'IO.inspect(CCPrecompiler.all_supported_targets(:fetch))'
• git diff --check
• mix test

Note: I validated the normal native build/test path locally on macOS. The Android cross-compile path depends on the GitHub-hosted Linux runner's Android NDK installation, so the new workflow is the first full NDK validation point.