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/ios/Flutter/AppFrameworkInfo.plist

@@ -20,7 +20,5 @@
   <string>????</string>
   <key>CFBundleVersion</key>
   <string>1.0</string>
-  <key>MinimumOSVersion</key>
-  <string>8.0</string>
 </dict>
 </plist>