Android APK packages

Android SDK, APK, and Maven-cache helpers live in harbor-android.

harbor-rs.lib re-exports mkAndroidSdk, mkAndroidDevShell, mkAndroidApk, mkAndroidApkDevBuilder, mkAndroidFlavorTable, and findLocalMavenCache for one migration release. New consumers should take harbor-android directly.

Keep mkToolchain here and pass rustToolchain into the Android helpers. mkGradlePackage and fetchMavenCache also stay in harbor-rs.