Add namespace to example app build.gradle

Add namespace declaration required by AGP 8.6+. The namespace
replaces the package attribute from AndroidManifest.xml.

Fixes: "Namespace not specified. Specify a namespace in the module's
build file"

Author: claude

example/android/app/build.gradle

@@ -28,6 +28,7 @@ if (flutterVersionName == null) {
 }
 
 android {
+    namespace "com.nschairer.keyboard_height_plugin_example"
     compileSdkVersion 34
     ndkVersion flutter.ndkVersion