Change GetService call to GetRequiredService

Remove the assertion that the returned service is not null, since the
GetRequiredService extension method will throw instead of ever
returning null.
Fix Win7 vs Win8 detection logic for SP800-108-CTR-HMACSHA512.
Allow DpapiNGXmlEncryptor to get the current identity's SID on CoreCLR.
DataProtectionServices should use keys stored in HKLM auto-gen registry when running on IIS without user profile.
Fix ManagedAuthenticatedEncryptor not round-tripping payloads correctly.