feat: Implement per-app refresh rate

[Mudit200408]

No description provided.

[sameerasw]

Hi! Can you take a look at these concerns?

Potential Issues & Suggestions

1. Shell Execution Overhead on Main Thread during App Switching

• Location: AppFlowHandler.kt lines in checkPerAppRefreshRate()
• Issue: RefreshRateUtils.applyFixedRefreshRate() and applyDynamicRefreshRate() run ShellUtils.runCommand(...) directly on the thread calling checkPerAppRefreshRate().
• Detail: ShellUtils.runCommand executes su / shizuku shell commands synchronously. Running up to 4 shell commands (settings put system..., settings put global...) per app switch can freeze window transitions or cause noticeable UI latency during app launches.
• Recommendation: Run RefreshRateUtils shell commands inside a background CoroutineScope(Dispatchers.IO) worker thread to prevent main thread blocking.

2. State Leak if Accessibility / App Detection Service is Stopped

• Location: AppFlowHandler.kt
• Issue: If the service crashes or is stopped while a per-app refresh rate is active, restoreFromSnapshot() is never called, leaving the device locked at the custom refresh rate (e.g. 60Hz fixed).
• Recommendation: Implement a cleanup hook on service destroy (onDestroy()) that checks if perAppRateSnapshot != null and invokes restoreFromSnapshot().