Make the AndroidContext superclass destructor virtual (flutter#22555)

Also remove an obsolete external_view_embedder reference from
AndroidSurfaceFactoryImpl.

Fixes flutter/flutter#70621

Author: jason-simmons

shell/platform/android/context/android_context.h

@@ -22,7 +22,7 @@ class AndroidContext {
  public:
   explicit AndroidContext(AndroidRenderingAPI rendering_api);
 
-  ~AndroidContext();
+  virtual ~AndroidContext();
 
   AndroidRenderingAPI RenderingApi() const;
 

shell/platform/android/platform_view_android.h

@@ -35,7 +35,6 @@ class AndroidSurfaceFactoryImpl : public AndroidSurfaceFactory {
  private:
   const AndroidContext& android_context_;
   std::shared_ptr<PlatformViewAndroidJNI> jni_facade_;
-  std::weak_ptr<AndroidExternalViewEmbedder> external_view_embedder_;
 };
 
 class PlatformViewAndroid final : public PlatformView {