Start rental clock when nonce-free offline license loaded

Merge from Widevine repo of http://go/wvgerrit/103107

When an offline license is reloaded, if it does not have a usage entry
to indicate when the rental clock was started, the start time defaults
to 0 in the ODK library (in OEMCrypto). This CL changes the code to
start the rental clock in this case. It does this by signing a dummy
message, which triggers the ODK library to start the rental clock.

Bug: 161585265
Bug: 161023174
Test: GTS tests. http://go/forrest-run/L55100000642199761
Change-Id: I4cf555b2fb43009ffb62e7b2c1a37265c3f70bfe

libwvdrmengine/cdm/core/src/cdm_session.cpp

@@ -286,6 +286,15 @@ CdmResponseType CdmSession::RestoreOfflineSession(const CdmKeySetId& key_set_id,
             key_response_, &provider_session_token) ||
         usage_table_header_ == nullptr) {
       provider_session_token.clear();
+      // TODO(b/161023174): remove this code in v17.
+      std::string fake_message("empty message");
+      std::string core_message;
+      std::string license_request_signature;
+      // Sign a fake message so that OEMCrypto will start the rental clock. The
+      // signature and generated core message are ignored.
+      CdmResponseType status = crypto_session_->PrepareAndSignLicenseRequest(
+          fake_message, &core_message, &license_request_signature);
+      if (status != NO_ERROR) return status;
     } else if (!VerifyOfflineUsageEntry()) {
       LOGE("License usage entry is invalid, cannot restore");
       return LICENSE_USAGE_ENTRY_MISSING;