keyboard(android): pre-R IME tracking + flush dirty marks during persistent callback

KeyboardPlugin.setupInsetsCallback used to early-return on SDK < R, so
the FFI height stayed at 0 on API 29 devices like the Huawei Mate 20 Pro
— the chat composer never tracked the IME. Run the global-layout
listener on all SDKs, and on pre-R also wire setOnApplyWindowInsetsListener
since EMUI 10's IME-hide dispatches new insets without a follow-up layout
pass. Pre-R IME height comes from systemWindowInsetBottom −
stableInsetBottom (stable insets exclude things that animate in/out).

Inside XKeyboard._onFrame, follow notifyListeners with an explicit
scheduleFrame. _onFrame runs as a persistent frame callback after the
build phase has finished, so setState in listeners marks elements dirty
but ensureVisualUpdate is a no-op in this phase — the steady-state pump
masked the issue while the keyboard was open but on the close-edge
(h transitions to 0) the pump stops and the final rebuild was never
scheduled.

lib/src/keyboard.dart

@@ -229,9 +229,16 @@ class XKeyboard with ChangeNotifier {
     if ((h - _height).abs() > 0.5) {
       _height = h;
       notifyListeners();
+      // notifyListeners triggers setState in listeners, which marks elements
+      // dirty. We're inside a persistent frame callback (after the build phase
+      // has already run), so `ensureVisualUpdate` is a no-op — without an
+      // explicit scheduleFrame the dirty marks never get flushed. This matters
+      // especially on the down-edge (h transitions to 0) where the steady-state
+      // pump below stops scheduling.
+      SchedulerBinding.instance.scheduleFrame();
     }
 
-    // Schedule frames while the curve is still running.
+    // Steady-state pump while the curve is still running or the keyboard is up.
     final curveActive = _isAnimating &&
         (ts - _animStartTime) < _animDuration;
     if (curveActive || (!Platform.isIOS && (h > 0 || _height > 0))) {