Changes JSON format for provisioning request and response to match server change.

This new format uses the SignedProvisioningMessage proto buffer definition so
the client does not have to parse the message and signature from the JSON
response separately. This change makes it more flexible to extend the fields
in the SignedProvisioningMessage.
Adds Apiary API key to the default provisioning server url.
Fixes a bug in GetCertRequestResponse() where a LOGD() can generate a
fault if there is no response data.

Bug: 8620943

Merge of https://widevine-internal-review.googlesource.com/#/c/5230/
from Widevine CDM repository

Change-Id: I4945ee2d16f88666e41edf990dd07102a9271105

libwvdrmengine/cdm/core/include/cdm_engine.h

@@ -102,7 +102,6 @@ class CdmEngine : public TimerHandler {
   bool CancelSessions();
   void CleanupProvisioningSession(const CdmSessionId& cdm_session_id);
   void ComposeJsonRequest(const std::string& message,
-                          const std::string& signature,
                           CdmProvisioningRequest* request);
 
   // Parse a blob of multiple concatenated PSSH atoms to extract the first