Store key set ID with usage info

[ Merge of http://go/wvgerrit/16241 and http://go/wvgerrit/16364 ]

This will allow a usage session to be loaded later by key set ID.
This is needed for EME-style secure stop in the new CE CDM API.

b/25816911

Change-Id: I916340047492fbc0556d0e90bd2eac0f3eafe597

libwvdrmengine/cdm/core/src/device_files.proto

@@ -46,6 +46,7 @@ message UsageInfo {
     optional bytes token = 1;
     optional bytes license_request = 2;
     optional bytes license = 3;
+    optional bytes key_set_id = 4;
   }
 
   repeated ProviderSession sessions = 1;