Modernize build for Android API 36 + NDK 27, fix JNI class loader SIGABRT

[sharpninja]

Summary

This PR brings android-spatialite up to date with current Android tooling (API 36, NDK r27, Gradle 8, AGP 8.7) and fixes a fatal SIGABRT that occurs when the library is loaded by non-standard runtimes such as .NET Android (Mono/Xamarin).

Build system modernization

• AGP: 3.6.0-alpha06 → 8.7.3
• Gradle: 5.6 → 8.11.1
• compileSdk / targetSdk: 29 → 36
• minSdk: 16 → 21 (matches current Google Play requirements)
• Java compatibility: 1.7 → 1.8
• Repository: replaced deprecated jcenter() with mavenCentral()
• settings.gradle: added pluginManagement block (required by Gradle 8)
• AndroidX dependencies: updated to current stable releases
• SQLite amalgamation: updated to 3.49.1 (January 2025)
• Misc: replaced deprecated GradleScriptException with GradleException

NDK / native changes

Setting	Before	After	Why
APP_STL	c++_shared	c++_static	Eliminates libc++_shared.so APK dependency; avoids version conflicts when multiple native libraries ship in the same app
APP_PLATFORM	android-16	android-21	Aligns with minSdk 21
APP_ABI	armeabi-v7a,arm64-v8a,x86,x86_64	armeabi-v7a arm64-v8a x86_64	Drops x86 (virtually unused on real devices); space-separated per NDK r23+ convention
APP_DEPRECATED_HEADERS	true	removed	Deprecated and unnecessary with NDK r27
APP_SHORT_COMMANDS	true	NDK_APP_SHORT_COMMANDS = true	Corrected variable name

Added -Wno-incompatible-function-pointer-types to individual ndk-module .mk files where Clang 18+ (NDK r27) emits hard errors for legacy C function pointer casts in vendored source (freexl, geos, libiconv, liblzma, libspatialite, libxml2, proj.4).

JNI class loader fix (critical — SIGABRT)

Problem: When libandroid_spatialite.so is loaded by a non-standard runtime (e.g., .NET Android / Mono), System.loadLibrary fires before any Java application code runs. JNI_OnLoad therefore executes on the bootstrap class loader thread, where FindClass("org/spatialite/database/SQLiteCustomFunction") returns NULL. The existing code calls LOG_FATAL_IF(!clazz, ...) which triggers abort() → SIGABRT.

This is specific to runtimes that preload native libraries during VM initialization (before the app class loader is available). Standard Android Java/Kotlin apps are unaffected because SQLiteDatabase.<clinit> runs on an app thread.

Fix:

1. JNI_OnLoad now only saves the JavaVM* pointer — no RegisterNatives, no FindClass, no GetFieldID.
2. All four registration functions (register_android_database_SQLiteConnection, SQLiteGlobal, SQLiteDebug, CursorWindow) are called exactly once from nativeOpen() via std::call_once, which is the first native method invoked on an actual Java application thread where the app class loader is available.
3. Replaced the FIND_CLASS macro with direct env->FindClass() + LOG_FATAL_IF in register_android_database_SQLiteConnection for clarity and debuggability.

This is fully backward-compatible: standard Java/Kotlin Android apps call nativeOpen() on an app thread identically to before; the lazy registration is transparent.

Test plan

• Built with NDK r27.2 (Clang 18) on Windows — all ABIs compile clean
• Verified .NET Android (Mono) app loads libandroid_spatialite.so without SIGABRT
• Verified nativeOpen() succeeds and RegisterNatives completes on first database open
• Verified mod_spatialite extension loads via SELECT load_extension('mod_spatialite')
• Upstream maintainer verification on standard Java/Kotlin Android project

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