Move RegistryKey to shared CoreLib partition (#20067)

Move RegistryKey to shared CoreLib partition (#20067)

* Move RegistryKey to shared CoreLib partition

- Cut down RegistryKey to just what CoreLib needs. I went back and forth on whether to share the corefx implementation with ifdefs or not. I have choosen to duplicate it at the end. The ifdefs were either too complex or there was too much cruft left behind that the IL linker was not able to remove.
- Move the internal CoreLib implementation of Registry to Internal.Win32 namespace to ensure that it is not confused with the public standlone one

Fixes #10741 and #17899