Make AndroidContext::IsValid virtual (#22665)

Fixes https://github.com/flutter/flutter/issues/70938

shell/platform/android/android_context_gl.h

@@ -103,7 +103,7 @@ class AndroidContextGL : public AndroidContext {
   /// @return     Whether the current context is valid. That is, if the EGL
   ///             contexts were successfully created.
   ///
-  bool IsValid() const;
+  bool IsValid() const override;
 
   //----------------------------------------------------------------------------
   /// @return     Whether the current context was successfully clear.

shell/platform/android/context/android_context.h

@@ -26,7 +26,7 @@ class AndroidContext {
 
   AndroidRenderingAPI RenderingApi() const;
 
-  bool IsValid() const;
+  virtual bool IsValid() const;
 
  private:
   const AndroidRenderingAPI rendering_api_;