android: keyboard height tracking via JNI/FFI bridge

- C bridge (keyboard_bridge.c) stores keyboard state in globals.
  Kotlin writes via JNI, Dart reads via FFI — zero async delay,
  same architecture as iOS.
- WindowInsetsAnimation.Callback tracks open/close per-frame.
- OnGlobalLayoutListener catches silent height changes (emoji
  keyboard resize, floating keyboard toggle).
- Dart animation replay stays iOS-only; Android reads native
  per-frame values directly.
- Cleaned up old Java stub, updated build.gradle for Kotlin + CMake
  with 16KB page alignment (Android 15+).
- Example app rewritten to demonstrate UxKeyboard usage.

lib/ux.dart

@@ -3,15 +3,4 @@ library ux;
 export 'src/bend_box.dart';
 export 'src/json_extension.dart';
 export 'src/bezier.dart';
-
-import 'dart:async';
-import 'package:flutter/services.dart';
-
-class UX {
-  static const MethodChannel _channel = const MethodChannel('ux');
-
-  static Future<String> get platformVersion async {
-    final String version = await _channel.invokeMethod('getPlatformVersion');
-    return version;
-  }
-}
+export 'src/keyboard.dart';