Re-enable CA1802 (use consts instead of readonly statics) (dotnet/corefx#39782)

Re-enable CA1802 (use consts instead of readonly statics) (dotnet/corefx#39782)

* Re-enable CA1802 (use consts instead of readonly statics)

I enabled it only for privates and internals, as we've previously violated this in public surface area, and as everything goes through API review moving forward, didn't seem worthwhile fighting it and adding suppressions in various places.

* Address PR feedback

Commit migrated from https://github.com/dotnet/corefx/commit/318b5941ba4e713155d82c674c7f5bd0e68faf7b