Add discovery service to stable APIs

generated/googleapis/test/appsactivity/v1_test.dart

 library googleapis.appsactivity.v1.test;
 
 import "dart:core" as core;
 import "dart:collection" as collection;
 import "dart:async" as async;
 import "dart:convert" as convert;
 
 import 'package:http/http.dart' as http;
 import 'package:http/testing.dart' as http_testing;
 import 'package:unittest/unittest.dart' as unittest;
@@ skipping 521 matching lines @@
       res.list(drive_ancestorId: arg_drive_ancestorId, drive_fileId: arg_drive_fileId, groupingStrategy: arg_groupingStrategy, pageSize: arg_pageSize, pageToken: arg_pageToken, source: arg_source, userId: arg_userId).then(unittest.expectAsync(((api.ListActivitiesResponse response) {
         checkListActivitiesResponse(response);
       })));
     });
 
   });
 
 
 }