CDM workarounds for OEMCrypto issues

Merge of https://widevine-internal-review.googlesource.com/#/c/10614/
from the widevine cdm repo.

* b/15467844 - GenerateRSASignature returns OEMCrypto_ERROR_INVALID_CONTEXT
  when called with a non-NULL signature pointer and signature length of
  0 (rather than OEMCrypto_ERROR_SHORT_BUFFER)
* b/15989260 - OEMCrypto_DecryptCTR does not return OEMCrypto_ERROR_KEY_EXPIRED
  after keys have expired

Also addresses
* integration test updated to reflect that loading certificate errors are
  returned on OpenSession rather than GenerateKeyRequest
* compiler warning on type casting

b/15989261

Change-Id: Ib68b972651479e99b9d05de4493aac55a96c4f39

libwvdrmengine/cdm/core/src/crypto_session.cpp

@@ -24,6 +24,7 @@ std::string EncodeUint32(unsigned int u) {
   s.append(1, (u >> 0) & 0xFF);
   return s;
 }
+const uint32_t kRsaSignatureLength = 256;
 }
 
 namespace wvcdm {
@@ -570,6 +571,7 @@ bool CryptoSession::GenerateRsaSignature(const std::string& message,
   LOGV("GenerateRsaSignature: id=%ld", (uint32_t)oec_session_id_);
   if (!signature) return false;
 
+  signature->resize(kRsaSignatureLength);
   size_t length = signature->size();
   OEMCryptoResult sts = OEMCrypto_GenerateRSASignature(
       oec_session_id_, reinterpret_cast<const uint8_t*>(message.data()),