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