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/build.gradle

@@ -1,18 +1,7 @@
-buildscript {
-    repositories {
-        google()
-        jcenter()
-    }
-
-    dependencies {
-        classpath 'com.android.tools.build:gradle:3.2.1'
-    }
-}
-
 allprojects {
     repositories {
         google()
-        jcenter()
+        mavenCentral()
     }
 }
 
@@ -24,6 +13,6 @@ subprojects {
     project.evaluationDependsOn(':app')
 }
 
-task clean(type: Delete) {
-    delete rootProject.buildDir
+tasks.register("clean", Delete) {
+    delete rootProject.layout.buildDirectory
 }