Make ResourceManager netstandard 2.0 API behave correctly (#12536)

Make ResourceManager netstandard 2.0 API behave correctly (#12536)

* Make ResourceManager netstandard 2.0 API behave correctly

In netstandard 2.0 we have enabled the APIs like GetObject, GetStream,..etc. these are not functioning correctly in UWP apps because was assuming we always uses PRI files. The changes here is to initialize the managed resources too and let these APIs behave as if we are running inside desktop app.

* added #ifdef

* Remove the throwing as PRI can handle case insenitive lookup

* remove un-needed resources