Introduce UsageTableHeader class

[ Merge of http://go/wvgerrit/23820 ]

The UsageTableHeader class is a singleton that CDM sessions will share.
A separate object will be created for each security level. The class
synchronizes access to usage table header and associated data-structures
and controls when they are read in or written out to non-secure persistent
storage. Upgrades from a fixed size usage table (supported by previous
versions of the OEMCrypto API v9-12) are handled by this class.

b/34327459

Test: Verified by unit/integration tests on angler

Change-Id: Ifc5996985e76bc260c01e55bc12aab1248389a80

libwvdrmengine/cdm/core/include/license.h

@@ -62,6 +62,10 @@ class CdmLicense {
     return is_offline_;
   }
 
+  static bool ExtractProviderSessionToken(
+      const CdmKeyResponse& license_response,
+      std::string* provider_session_token);
+
  private:
 
   CdmResponseType HandleKeyErrorResponse(