Correct KeySetId value when returned by AddKey

[ Merge of http://go/wvgerrit/25643 ]

The MediaDrm#provideKeyResponse API states that an empty byte array is
returned when the license type is streaming or release but a non-empty
value was being returned in some cases.

The KeySetId is now returned when the license type is offline or when
the license is streaming and has a secure stop associated with it.

Test: Verified by request_license_test integration tests. Tests have been
      modified to validate the returned Key Set Id values.

b/36093612

Change-Id: I82dba537c77ddd1d1876cbce58729f3db901ee51

libwvdrmengine/cdm/core/include/cdm_session.h

@@ -117,6 +117,10 @@ class CdmSession {
   virtual bool is_offline() { return is_offline_; }
   virtual bool is_temporary() { return is_temporary_; }
   virtual bool license_received() { return license_received_; }
+  virtual bool has_provider_session_token() {
+    return (license_parser_.get() != NULL &&
+        license_parser_->provider_session_token().size() > 0);
+  }
 
   virtual CdmUsageSupportType get_usage_support_type()
       { return usage_support_type_; }