Merge OEMCrypto KDF and usage functions

Since KDF functions are only used right before specific functions, this
merges them to simplify internal state within OEMCrypto.

Fixes: 299527712
Change-Id: I426cfcdc102bd73cf65cd809b213da2474f44b34

libwvdrmengine/oemcrypto/test/oemcrypto_cast_test.cpp

@@ -49,7 +49,6 @@ TEST_F(OEMCryptoLoadsCertificateAlternates, ForbidUseAsDRMCert) {
   if (key_loaded_) {
     // The other padding scheme should fail.
     DisallowForbiddenPaddingDRMKey(kSign_RSASSA_PSS, 83);
-    DisallowDeriveKeys();
   }
 }