Break the reference cycle between the surface factory and the external view embedder

See flutter/flutter#68315

Author: jason-simmons

shell/platform/android/platform_view_android.cc

@@ -107,7 +107,9 @@ PlatformViewAndroid::PlatformViewAndroid(
       jni_facade_(jni_facade),
       platform_view_android_delegate_(jni_facade) {}
 
-PlatformViewAndroid::~PlatformViewAndroid() = default;
+PlatformViewAndroid::~PlatformViewAndroid() {
+  surface_factory_->SetExternalViewEmbedder(nullptr);
+}
 
 void PlatformViewAndroid::NotifyCreated(
     fml::RefPtr<AndroidNativeWindow> native_window) {