Fix culture handling in legacy System.Json library (dotnet/corefx#34811)

Fix culture handling in legacy System.Json library (dotnet/corefx#34811)

* Fix System.Json code using NumberFormatInfo.InvariantInfo

The intention here was obviously to use invariant culture, but NumberFormatInfo will be ignored for various primitive types that don't respect NFI, e.g. TimeSpan, which respects DateTimeFormatInfo.

* Fix handling of culture in System.Json tests

In particular, CultureInfo.CurrentCulture should not be modified in-process in tests, as on UWP it bleeds across threads.

Commit migrated from https://github.com/dotnet/corefx/commit/f398b6f7c3d08d8e437939cbd9ef29cb3beda1db