navigationStackIsEmpty method to enable predictive back

shell/platform/android/io/flutter/embedding/android/FlutterActivity.java

@@ -666,14 +666,21 @@ protected void onCreate(@Nullable Bundle savedInstanceState) {
   public void registerOnBackInvokedCallback() {
     if (Build.VERSION.SDK_INT >= 33) {
       // TODO(justinmc): This is really the one and only thing that stops root
-      // pback for me.
+      // pback for me. There are probably other places you need to mess with
+      // this for other embedding strategies!
       Log.e("justin", "registerOnBackInvokedCallback");
       getOnBackInvokedDispatcher()
           .registerOnBackInvokedCallback(
               OnBackInvokedDispatcher.PRIORITY_DEFAULT, onBackInvokedCallback);
     }
   }
 
+  @Override
+  public void navigatorIsEmpty() {
+    Log.e("justin", "navigatorIsEmpty in embedding FlutterActivity");
+    unregisterOnBackInvokedCallback();
+  }
+
   /**
    * Unregisters the callback from OnBackInvokedDispatcher.
    *

shell/platform/android/io/flutter/embedding/android/FlutterFragment.java

@@ -1661,6 +1661,13 @@ public boolean popSystemNavigator() {
     return false;
   }
 
+  @Override
+  public void navigatorIsEmpty() {
+    // TODO(justinmc): Implement. Implementing in FlutterActivity first.
+    // unregisterOnBackInvokedCallback();
+    Log.e("justin", "navigatorIsEmpty in embedding FlutterFragment");
+  }
+
   @VisibleForTesting
   @NonNull
   boolean shouldDelayFirstAndroidViewDraw() {

shell/platform/android/io/flutter/plugin/platform/PlatformPlugin.java

@@ -56,6 +56,8 @@ public interface PlatformPluginDelegate {
      *     androidx.activity.OnBackPressedDispatcher} will be executed.
      */
     boolean popSystemNavigator();
+
+    void navigatorIsEmpty();
   }
 
   @VisibleForTesting
@@ -111,6 +113,7 @@ public void setSystemUiOverlayStyle(
 
         @Override
         public void navigatorIsEmpty() {
+          Log.e("justin", "navigatorIsEmpty in PlatformHandler.PlatformMessageChannel.");
           PlatformPlugin.this.navigatorIsEmpty();
         }
 
@@ -482,8 +485,7 @@ private void setSystemChromeSystemUIOverlayStyle(
 
   private void navigatorIsEmpty() {
     Log.e("justin", "navigatorIsEmpty in PlatformPlugin");
-    // TODO(justinmc): Can't quite call it like this, need FlutterActivity.
-    // activity.unregisterOnBackInvokedCallback();
+    platformPluginDelegate.navigatorIsEmpty();
   }
 
   private void popSystemNavigator() {