Introduce unified notification support and use foreground notifications for core test tasks. Added NotificationChannelType enum and NotificationHelper (cached NotificationManager and Builder cache, channel creation, notify/update/startForeground/stopForeground/cancel). Updated AndroidManifest to mark CoreTestService as a specialUse foreground service. Renamed AppConfig message constants (MSG_MEASURE_CONFIG_START, MSG_MEASURE_CONFIG_CANCEL, MSG_MEASURE_CONFIG_SUCCESS) and updated usages: MessageUtil now starts a foreground service on O+ for measurement start and handles cancel without forcing start; CoreTestService starts foreground immediately, logs lifecycle, manages RealPingWorkerService via onProgress/onFinish callbacks, and stops foreground when done; RealPingWorkerService reports progress via onProgress instead of direct UI messages; SubscriptionUpdater now uses NotificationHelper and NotificationChannelType for update notifications. These changes improve notification performance, ensure startForegroundService compliance on Android O+, and centralize notification logic.
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