Add cake-addin tag to Cake.Android.SdkManager NuGet package

The [NuGet Gallery](https://nuget.org) will soon include a "Cake" tab with instructions on how to use NuGet packages in Cake build scripts. I'd like to suggest that you include the tag `cake-addin` in the NuGet package of this addin, so that the NuGet Gallery can display the correct instructions to install this addin.

The UI will look similar to this:


Documentation on best practices on tags for Cake addins: https://cakebuild.net/docs/extending/addins/best-practices#tags

Start by locating the NuGet package metadata and packaging configuration in the repository, then compare the current package tags with the linked Cake add-in tagging guidance. The work is done when the published Cake.Android.SdkManager package includes the `cake-addin` tag and the package metadata remains valid.

Written by the indexing model from the issue text.