Add support for .NET 6 run-time configuration settings for mobile targets

.NET Core support setting application configuration using various mechanisms as described at https://docs.microsoft.com/en-us/dotnet/core/run-time-config/. Such settings need to be correctly propagated to [AppContext](https://docs.microsoft.com/en-us/dotnet/api/system.appcontext) class as well as to internal runtime variables for mobile applications.