Make CdmProvisioningResponse const and DeviceFiles level support

* Make CdmProvisioningResponse const.

  [ Merge of http://go/wvgerrit/14618 ]

  The lack of const on this reference seems to be a mistake, since the
  responses is never modified.  This also allows the new CE CDM to pass
  responses directly through from the caller.

* Let Properties determine DeviceFiles level support

  [ Merge of http://go/wvgerrit/14620 ]

  Non-Android platforms do not have multiple security levels, and so do
  not use the security level to construct a base path.

  Instead of requiring a known "security level" to construct a file,
  accept anything that platform Properties will accept as a base path.

* Drop Properties::GetSecurityLevel().

  [ Merge of http://go/wvgerrit/14617 ]

  This seems to be dead code.

Change-Id: I94a970279213100730d6e6c763558dbe386f936a

libwvdrmengine/cdm/core/include/properties.h

@@ -58,8 +58,6 @@ class Properties {
   static bool GetSecurityLevelDirectories(std::vector<std::string>* dirs);
   static bool GetApplicationId(const CdmSessionId& session_id,
                                std::string* app_id);
-  static bool GetSecurityLevel(const CdmSessionId& session_id,
-                               std::string* security_level);
   static bool GetServiceCertificate(const CdmSessionId& session_id,
                                     std::string* service_certificate);
   static bool UsePrivacyMode(const CdmSessionId& session_id);