Remove a few initialization allocations from Cng (dotnet/corefx#41373)

Remove a few initialization allocations from Cng (dotnet/corefx#41373)

Save four "startup" allocations in System.Security.Cryptography.Cng.

- Appending the '\0' is better done at the call site, because then the concat is done with two constants at compile time.
- There's no benefit to using ToCharArray instead of just passing in the string.
- Once we get rid of those, we can just inline the construction into the declarations.

Commit migrated from https://github.com/dotnet/corefx/commit/296c0e71ddcc4885d30af18dea829c1923cfdc5c