✨ Add Xamarin support (Android) to C# Wrapper

## Feature Request

**Description**
Xamarin support was temporarily removed but should still be supported under the .NET Standard implementations.
This may require some specialized building of the package for these particular runtimes and more complicated setup so that a developer can easily utilize the library.

The suggestion is to implement Conditions to implement some of the previous code originally in the previously made iOS and Android projects. This may require using [Build Extras](https://github.com/novotnyllc/MSBuildSdkExtras).

-[Android Specifics](https://github.com/microsoft/electionguard-cpp/blob/0bad662f3c6c32f4f7c257b2826d448930736594/bindings/netstandard/ElectionGuard/ElectionGuard.Android/ElectionGuard.Android.csproj#L52)

See

- [ ] Add Android binaries into runtimes and project
- [ ] Update documentation after Android and iOS support
- [ ] Build test project to demonstrate usage but do not commit within PR.

**Helpful Links**
https://docs.microsoft.com/en-us/nuget/create-packages/multiple-target-frameworks-project-file
https://docs.microsoft.com/en-us/nuget/create-packages/creating-a-package-msbuild
https://docs.microsoft.com/en-us/dotnet/standard/frameworks
https://github.com/edsnider/latestversionplugin/blob/master/src/Xam.Plugin.LatestVersion.csproj
https://docs.microsoft.com/en-us/nuget/create-packages/supporting-multiple-target-frameworks

首先将所引用的 ElectionGuard.Android.csproj 与当前的 bindings/netstandard 项目进行比较，然后检查 runtime 和 Android 二进制文件是如何打包的。创建一个本地 Xamarin Android 测试项目，以验证其使用方式。完成的标准是包含 Android 二进制文件、项目能够针对目标 runtime 构建、文档已更新，并且示例项目不被提交。

由索引模型根据 Issue 内容生成。