Xamarin allows you to create business logic in C# and use them for both Android / iOS / Windows Phone. You have to write your view logic also in C#. Xamarin has ported the API for both iOS and Android to C#. You can still use the designer tools from xCode (I guess for Android as well) and use them in your project. But it's true you'll have to use Visual Studio or Mono Develop to create your app.
So you either use the xamarin solution or you'll have to create a cross-platform library in C/C++. For iOS you can indeed just add C++ code. For Android you'll have to use the NDK. You'll have to use JNI (Java Native Interface) to be able to let the Java and C++ code talk to each other.
NDK: https://developer.android.com/tools/sdk/ndk/index.html
JNI Tutorial: https://code.google.com/p/awesomeguy/wiki/JNITutorial