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.

ios/Classes/UxPlugin.h

@@ -1,4 +0,0 @@
-#import <Flutter/Flutter.h>
-
-@interface UxPlugin : NSObject<FlutterPlugin>
-@end

ios/Classes/UxPlugin.m

@@ -1,20 +0,0 @@
-#import "UxPlugin.h"
-
-@implementation UxPlugin
-+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
-  FlutterMethodChannel* channel = [FlutterMethodChannel
-      methodChannelWithName:@"ux"
-            binaryMessenger:[registrar messenger]];
-  UxPlugin* instance = [[UxPlugin alloc] init];
-  [registrar addMethodCallDelegate:instance channel:channel];
-}
-
-- (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
-  if ([@"getPlatformVersion" isEqualToString:call.method]) {
-    result([@"iOS " stringByAppendingString:[[UIDevice currentDevice] systemVersion]]);
-  } else {
-    result(FlutterMethodNotImplemented);
-  }
-}
-
-@end