Reduce Windows_arm64 plugin_test_windows test timeout (#145110)

Windows Defender sometimes kills the `Windows_arm64 plugin_test_windows` test process, causing the test to hang until it times out after 30 minutes. This reduces the test timeout to 900 seconds (15 minutes) to recover from this scenario faster.

Test timeout duration was chosen by looking at successful duration percentiles in the last 100 days:

duration_seconds_p90 | duration_seconds_p99 | duration_seconds_max
-- | -- | --
532 | 545 | 576

BigQuery SQL:

```sql
WITH
  successful_steps AS (
    SELECT
      b.id,
      TIMESTAMP_DIFF(s.end_time, s.start_time, SECOND) AS duration_seconds,
    FROM cr-buildbucket.flutter.builds AS b, UNNEST(steps) AS s
    WHERE
      create_time > TIMESTAMP_SUB(CURRENT_TIMESTAMP(), INTERVAL 100 DAY)
      AND regexp_substr(input.gitiles_commit.project, '[^\\/]+$') = 'flutter'
      AND builder.project || '/' || builder.bucket || '/' || builder.builder
        = 'flutter/prod/Windows_arm64 plugin_test_windows'
      AND name = 'run plugin_test_windows'
      AND s.status = 'SUCCESS'
  )
SELECT
  percentiles[offset(90)] AS duration_seconds_p90,
  percentiles[offset(99)] AS duration_seconds_p99,
  duration_seconds_max
FROM (
  SELECT
    APPROX_QUANTILES(duration_seconds, 100) AS percentiles,
    MAX(duration_minutes) AS duration_seconds_max
  FROM successful_steps
);
```

`test_timeout_secs` is documented here: https://github.com/flutter/cocoon/blob/main/CI_YAML.md

Part of https://github.com/flutter/flutter/issues/145072

.ci.yaml

@@ -5335,6 +5335,7 @@ targets:
       tags: >
         ["devicelab", "hostonly", "windows", "arm64"]
       task_name: plugin_test_windows
+      test_timeout_secs: "900" # 15 minutes
     runIf:
       - dev/**
       - packages/flutter_tools/**