Add automatic Android NDK provisioning

[subham8907]

@bonede This PR makes Android native builds bootstrap the Android NDK automatically when no local NDK is configured.

What changed

• Add a downloadAndroidNdk Gradle task that downloads the configured Android NDK version into build/android-ndk.
• Make buildNative depend on that task when treeSitterTargets includes Android.
• Keep support for existing ANDROID_NDK_HOME, ANDROID_NDK_ROOT, ANDROID_HOME, and ANDROID_SDK_ROOT setups.
• Add Windows host compiler-name handling and document the Android build behavior.

Why

Android native builds currently fail on machines without a preconfigured NDK. This keeps installed NDKs working while allowing a fresh checkout to provision the required toolchain through Gradle.

Validation

• ./gradlew tasks --all --dry-run
• ./gradlew :tree-sitter:buildNative --dry-run
• ./gradlew :tree-sitter:build -PtreeSitterTargets=x86_64-macos
• ./gradlew :buildSrc:test
• Confirmed the configured Android NDK archive URL returns HTTP 200.

[subham8907]

@bonede This is the final PR for the Android NDK auto-provisioning change. It keeps existing local NDK setups working and adds a Gradle-managed fallback for fresh checkouts.