Enable time zone for netstandard1.7 (dotnet/coreclr#8074)

Enable time zone for netstandard1.7 (dotnet/coreclr#8074)

* Expose and reimplement TimeZone class

The change is exposing the missing TimeZone APIs from corelib so can be useded in netstandard 1.7.
The change here is re-implementing CurrentTimeZone class to be simpler and also to make it work for Linux platforms. we used TimeZoneInfo class to reimplement it and also cleaned up all used internal calls as not needed anymore.

* Small delta fix

* Fix the code according to the review comments

Commit migrated from https://github.com/dotnet/coreclr/commit/d3ad5db7bf3209a5b0ea341cee19230d2ba0fef5