Fix compiler warnings.

[jpobst]

Fix some NRT warnings that have crept in.

---

Also add <NeutralLanguage>en</NeutralLanguage> to fix:

Warning CA1824: Mark assemblies with NeutralResourcesLanguageAttribute

Basically this says that the "neutral" language this assembly contains is also the English translations, so we don't need to spend time probing for satellite resource assemblies when the English language is requested.

---

Also add <MSBuildWarningsAsMessages>NU1702</MSBuildWarningsAsMessages> to suppress:

Warning NU1702: ProjectReference 'D:\a\1\s\src\java-interop\java-interop.csproj' was resolved using 
'.NETFramework,Version=v4.7.2' instead of the project target framework '.NETStandard,Version=v2.0'. 
This project may not be fully compatible with your project.