Marks Linux_android opacity_peephole_grid_of_alpha_savelayers_perf__e2e_summary to be unflaky (#135960)

<!-- meta-tags: To be used by the automation script only, DO NOT MODIFY.
{
  "name": "Linux_android opacity_peephole_grid_of_alpha_savelayers_perf__e2e_summary"
}
-->
The issue https://github.com/flutter/flutter/issues/135118 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%20opacity_peephole_grid_of_alpha_savelayers_perf__e2e_summary%22).
This test can be marked as unflaky.