keyboard: fix interactive dismiss race conditions, rewrite example

Fix several race conditions in the iOS interactive dismiss flow:
- Don't skip keyboard close notifications during pan tracking, which
  left Dart unaware the keyboard was dismissed
- Guard resignFirstResponder with generation check so reopened keyboards
  aren't killed by stale dismiss completions
- Block pan tracking from starting during an in-flight dismiss animation
- Always reset keyboard view bounds when the keyboard opens, not just
  when the dismiss animator is still running
- Handle duration=0 keyboard notifications by snapping immediately
- Gate adaptive learning debug output behind kDebugMode

Rewrite the example as a chat UI demonstrating ListenableBuilder,
scroll freeze during interactive dismiss, and proper bottom inset
handling with max(keyboardHeight, safeBottom).

Modernize example Android project from v1 to v2 embedding with
current AGP/Gradle versions.

example/android/settings.gradle

@@ -1,15 +1,24 @@
-include ':app'
+pluginManagement {
+    def flutterSdkPath = {
+        def properties = new Properties()
+        file("local.properties").withInputStream { properties.load(it) }
+        def flutterSdkPath = properties.getProperty("flutter.sdk")
+        assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
+        return flutterSdkPath
+    }()
 
-def flutterProjectRoot = rootProject.projectDir.parentFile.toPath()
+    includeBuild("$flutterSdkPath/packages/flutter_tools/gradle")
 
-def plugins = new Properties()
-def pluginsFile = new File(flutterProjectRoot.toFile(), '.flutter-plugins')
-if (pluginsFile.exists()) {
-    pluginsFile.withReader('UTF-8') { reader -> plugins.load(reader) }
+    repositories {
+        google()
+        mavenCentral()
+        gradlePluginPortal()
+    }
 }
 
-plugins.each { name, path ->
-    def pluginDirectory = flutterProjectRoot.resolve(path).resolve('android').toFile()
-    include ":$name"
-    project(":$name").projectDir = pluginDirectory
+plugins {
+    id "dev.flutter.flutter-plugin-loader" version "1.0.0"
+    id "com.android.application" version "8.7.0" apply false
 }
+
+include ":app"