Marks Windows_android channels_integration_test_win to be unflaky (#135374)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Windows_android channels_integration_test_win"
}
-->
The issue https://github.com/flutter/flutter/issues/134636 has been closed, and the test has been passing for [50 consecutive runs](https://data.corp.google.com/sites/flutter_infra_metrics_datasite/flutter_check_test_flakiness_status_dashboard/?p=BUILDER_NAME:%22Windows_android%20channels_integration_test_win%22).
This test can be marked as unflaky.

.ci.yaml

@@ -5173,7 +5173,6 @@ targets:
       task_name: basic_material_app_win__compile
 
   - name: Windows_android channels_integration_test_win
-    bringup: true # Flaky https://github.com/flutter/flutter/issues/134636
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60