feat: Add BroadcastReceiver with KeepAliveService for reliable automation

[whjstc]

🎯 Problem

ExternalControlActivity causes screen flash/popup on some ROMs (especially Flyme, MIUI, ColorOS) when triggered by automation tools like Tasker, breaking the "background automation" experience.

✅ Solution

This PR adds two components for reliable background automation:

1. ExternalControlReceiver (BroadcastReceiver)

• Receives broadcasts from Tasker/automation tools
• Completely background operation - no UI triggered
• Supports START/STOP/TOGGLE actions

2. KeepAliveService (Foreground Service)

Why this is needed: During testing, I discovered that BroadcastReceiver fails when the app enters deep sleep (CACHED_EMPTY state, procState=19). The system freezes the app and blocks broadcasts from third-party apps.

Solution: A lightweight foreground service that:

• Keeps the app alive to reliably receive broadcasts
• Uses IMPORTANCE_MIN notification channel (minimal user disturbance)
• Idle service with near-zero battery impact
• Automatically starts with the app

🧪 Testing

Tested extensively on Flyme with Tasker:

• ✅ 100% reliable after hours in background
• ✅ No screen flash or popup
• ✅ App remains responsive to broadcasts even in deep background
• ✅ Process not frozen (procState improved from 19→4, oom_adj from 955→50)
• ✅ Minimal battery impact (idle foreground service)

📱 Usage

Tasker Configuration

Action: Send Intent

• Action: com.github.metacubex.clash.meta.action.START_CLASH (or STOP/TOGGLE)
• Target: Broadcast Receiver
• Package: com.github.metacubex.clash.meta
• Class: (leave empty)

Example Automation Scenarios

1. Auto-stop on home WiFi: Disable proxy when connected to trusted network
2. Auto-start on mobile data: Enable proxy when leaving home WiFi
3. Time-based control: Disable proxy during specific hours
4. Charging state: Enable proxy only when charging

📄 Files Changed

• app/src/main/java/.../ExternalControlReceiver.kt - BroadcastReceiver implementation
• app/src/main/java/.../KeepAliveService.kt - Foreground service for reliability
• app/src/main/java/.../MainApplication.kt - Auto-start KeepAliveService
• app/src/main/AndroidManifest.xml - Register receiver and service
• TASKER_GUIDE.md - Complete user guide with examples
• app/src/main/res/values/strings.xml - Notification strings

🔧 Technical Details

Why Foreground Service instead of battery optimization whitelist?

• Battery optimization is unreliable and ROM-dependent
• Foreground Service is the Android-official way to keep apps alive
• Used by all apps needing background reliability (music players, navigation, etc.)

Resource usage:

• CPU: 0% (idle service)
• Network: 0
• Battery: Negligible (no active operations)
• Memory: ~10-20MB (process kept in memory)

📚 Documentation

Complete Tasker setup guide included in TASKER_GUIDE.md with:

• Step-by-step configuration
• Common automation scenarios
• Troubleshooting tips
• FAQ