Move uap, uapaot, netfx, netcoreapp and netcoreapp20 constant definitions to top...

Move uap, uapaot, netfx, netcoreapp and netcoreapp20 constant definitions to top-level build file. (dotnet/corefx#31470)

* Move uap, uapaot, netfx and netcoreapp constant definitions to top-level build file.

* Trying to fix compilation of PerformanceCounterLib for netcoreapp2.0 targets.

* Change netcoreapp definition to mean the current version only.

* Fix conflict in System.Private.DataContractSerialization.csproj.

* Revert change in src/shims/manual/Directory.Build.props.

* Fix condition in test.

* Bring back TargetsNetCoreApp, it was lost during rebases.

* Move definition of netstandard to top-level build file. Remove more netcoreapp/uap definitions from individual files.

* Fix vbproj compatibility.

* Address PR review and unify the definitions to use TargetGroup directly.

* Add back one "netstandard" definition.

Commit migrated from https://github.com/dotnet/corefx/commit/ac994544e749eac3e85b60bf0012ae2f85ae0e49