Export Android platform vars from ndk-env

[StantonMatt]

Closes #198.

cargo ndk build already makes ANDROID_PLATFORM and ANDROID_ABI available to build scripts, but cargo ndk-env did not include them in its generated environment. This threads the selected platform and Android ABI through the shared environment builder so ndk-env, ndk-test, and the normal cargo command path use the same values.

I also added a focused regression test for the generated environment map.

Validation:

• cargo fmt --all -- --check
• cargo test build_env_exports_android_platform_and_abi --locked
• cargo test --locked
• cargo clippy -- -D warnings
• cargo install --locked --path . --root .codex-tmp/cargo-install/cargo-ndk
• cargo ndk-env --target arm64-v8a --platform 28 --json with a project-local fake NDK source.properties, checked ANDROID_PLATFORM=28, ANDROID_ABI=arm64-v8a, and CARGO_NDK_ANDROID_PLATFORM=28
• same fake-NDK smoke for default shell export output and --powershell
• git diff --check
• review-fix-loop: no discrete regression introduced by the patch

[bbqsrc]

If you could add another PR that makes the nightly clippy happy, that would be appreciated. 😄