Support 32 bytes session key

[ Merge of http://go/wvgerrit/149849 ]

With ECC based DRM cert, the session key is expected to be 32, as
compared to 16 bytes in RSA case. This CL adds supports for 32 bytes
session key.

Bug: 236317198
Test: oemcrypto_test
Change-Id: I657fdd92d17736a23375ddcd457f83efa6ca6d1f

libwvdrmengine/oemcrypto/test/ota_keybox_test.cpp

@@ -268,7 +268,7 @@ TEST_F(OTAKeyboxProvisioningTest, BasicTest) {
   std::copy(bit_size_string.begin(), bit_size_string.end(),
             std::back_inserter(enc_context));
   KeyDeriver keys;
-  keys.DeriveKeys(model_key.data(), mac_context, enc_context);
+  keys.DeriveKeys(model_key.data(), model_key.size(), mac_context, enc_context);
   const std::vector<uint8_t> message(
       request.data(),
       request.data() + request.size() - HMAC_SHA256_SIGNATURE_SIZE);