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.

pubspec.yaml

@@ -16,5 +16,9 @@ dev_dependencies:
 
 flutter:
   plugin:
-    androidPackage: io.swipelab.ux
-    pluginClass: UxPlugin
+    platforms:
+      ios:
+        pluginClass: KeyboardPlugin
+      android:
+        package: io.swipelab.ux
+        pluginClass: KeyboardPlugin