Marks Linux_android_emu android_defines_test to be unflaky (#159538)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_android_emu android_defines_test"
}
-->
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_emu%20android_defines_test%22).
This test can be marked as unflaky.

Author: fluttergithubbot

.ci.yaml

@@ -2202,7 +2202,6 @@ targets:
 
   - name: Linux_android_emu android_defines_test
     recipe: devicelab/devicelab_drone
-    bringup: true
     timeout: 60
     properties:
       tags: >