Make platforms optional during build.

we should add args to the C# build project to skip platforms we don't care about, specifically, the Android build will fail if the version of Unity you have installed is missing the Android module.

Start with the C# build project and the Android platform build path described in the issue. Determine how build arguments can select or skip platforms, then verify that a build succeeds when the Android module is unavailable and that requested platforms still build.

Written by the indexing model from the issue text.