Marks Linux_android flutter_gallery__start_up_delayed to be unflaky (#135565)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_android flutter_gallery__start_up_delayed"
}
-->
The issue https://github.com/flutter/flutter/issues/134631 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:%22Linux_android%20flutter_gallery__start_up_delayed%22).
This test can be marked as unflaky.

.ci.yaml

@@ -2106,7 +2106,6 @@ targets:
       task_name: flutter_gallery__start_up
 
   - name: Linux_android flutter_gallery__start_up_delayed
-    bringup: true # Flaky https://github.com/flutter/flutter/issues/134631
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60