[AppConfig-Perf] Fixed checkstyle issues for performance pacakges

[mssfang]

This PR fixes the checkstyle issues that reported by the @mrm9084 .

[INFO] --- maven-checkstyle-plugin:3.1.2:check (default) @ azure-data-appconfiguration-perf ---
[INFO] Starting audit...
[ERROR] C:\Users\mametcal\azure-sdk-for-java\sdk\appconfiguration\azure-data-appconfiguration-perf\src\main\java\com\azure\data\appconfiguration\perf\App.java:1: Missing package-info.java file. [JavadocPackage]
[ERROR] C:\Users\mametcal\azure-sdk-for-java\sdk\appconfiguration\azure-data-appconfiguration-perf\src\main\java\com\azure\data\appconfiguration\perf\core\ServiceTest.java:1: Missing package-info.java file. [JavadocPackage]
[ERROR] C:\Users\mametcal\azure-sdk-for-java\sdk\appconfiguration\azure-data-appconfiguration-perf\src\main\java\com\azure\data\appconfiguration\perf\ListConfigurationSettingsTest.java:23:5: Field "keyFilterSelector" is not assigned in constructor or methods.You should consider making the field final, or suppressing the warning. [EnforceFinalFields]
[ERROR] C:\Users\mametcal\azure-sdk-for-java\sdk\appconfiguration\azure-data-appconfiguration-perf\src\main\java\com\azure\data\appconfiguration\perf\ListConfigurationSettingsTest.java:25:5: Field "settingCount" is not assigned in constructor or methods.You should consider making the field final, or suppressing the warning. [EnforceFinalFields]
Audit done.
[INFO] ------------------------

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[mssfang]

/check-enforcer evaluate

[mssfang]

/check-enforcer override