Background service support on Android (#215)

* - Added support for BLE scanning from background services on Android.
- Enhanced `PermissionHandler` to be activity-aware, allowing silent success for already granted permissions when no activity is available.
- Updated README with best practices for requesting permissions in foreground before background operations.

* Simplify implementation

android/src/main/kotlin/com/navideck/universal_ble/PermissionHandler.kt

@@ -19,9 +19,17 @@ private const val TAG = "PermissionHandler"
  */
 class PermissionHandler(
     private val context: Context,
-    private val activity: Activity,
     private val requestCode: Int,
 ) {
+    private var activity: Activity? = null
+
+    /**
+     * Attaches or detaches an activity to the permission handler.
+     * Call with an Activity when attached, or null when detached.
+     */
+    fun attachActivity(activity: Activity?) {
+        this.activity = activity
+    }
     private var permissionRequestCallback: ((Result<Unit>) -> Unit)? = null
 
     /**
@@ -39,6 +47,10 @@ class PermissionHandler(
     /**
      * Requests the required Bluetooth permissions based on the manifest and Android version.
      *
+     * If activity is not available (e.g., running in a ForegroundTask), this method will
+     * check if all required permissions are already granted and succeed silently if so.
+     * It will only fail if permissions are needed but cannot be requested due to missing activity.
+     *
      * @param callback Called with the result of the permission request
      */
     fun requestPermissions(
@@ -52,7 +64,7 @@ class PermissionHandler(
             return
         }
 
-        // Check which permissions are declared in manifest
+        // Check which permissions need to be requested
         val permissionsToRequest = getRequiredPermissions(withFineLocation)
 
         if (permissionsToRequest.isEmpty()) {
@@ -61,6 +73,22 @@ class PermissionHandler(
             return
         }
 
+        // Permissions need to be requested - check if activity is available
+        val currentActivity = activity
+        if (currentActivity == null) {
+            // No activity available and permissions not granted
+            callback(
+                Result.failure(
+                    createFlutterError(
+                        UniversalBleErrorCode.FAILED,
+                        "Permissions not granted and activity is not available to request them. " +
+                        "Please request permissions while the app is in foreground."
+                    )
+                )
+            )
+            return
+        }
+
         // Check if we already have a pending permission request
         if (permissionRequestCallback != null) {
             callback(
@@ -76,7 +104,7 @@ class PermissionHandler(
 
         permissionRequestCallback = callback
         ActivityCompat.requestPermissions(
-            activity,
+            currentActivity,
             permissionsToRequest.toTypedArray(),
             requestCode
         )

android/src/main/kotlin/com/navideck/universal_ble/UniversalBlePlugin.kt

@@ -68,6 +68,7 @@ class UniversalBlePlugin : UniversalBlePlatformChannel, BluetoothGattCallback(),
         callbackChannel = UniversalBleCallbackChannel(flutterPluginBinding.binaryMessenger)
         context = flutterPluginBinding.applicationContext
         mainThreadHandler = Handler(Looper.getMainLooper())
+        permissionHandler = PermissionHandler(context, permissionRequestCode)
         bluetoothManager = context.getSystemService(Context.BLUETOOTH_SERVICE) as BluetoothManager
         safeScanner = SafeScanner(bluetoothManager)
 
@@ -86,6 +87,7 @@ class UniversalBlePlugin : UniversalBlePlatformChannel, BluetoothGattCallback(),
         context.unregisterReceiver(broadcastReceiver)
         callbackChannel = null
         mainThreadHandler = null
+        permissionHandler = null
     }
 
     override fun getBluetoothAvailabilityState(callback: (Result<Long>) -> Unit) {
@@ -1304,20 +1306,23 @@ class UniversalBlePlugin : UniversalBlePlatformChannel, BluetoothGattCallback(),
 
     override fun onAttachedToActivity(binding: ActivityPluginBinding) {
         activity = binding.activity
-        permissionHandler = PermissionHandler(context, binding.activity, permissionRequestCode)
         binding.addActivityResultListener(this)
         binding.addRequestPermissionsResultListener(this)
+        permissionHandler?.attachActivity(binding.activity)
     }
 
     override fun onDetachedFromActivity() {
         activity = null
-        permissionHandler = null
+        permissionHandler?.attachActivity(null)
+    }
+
+    override fun onDetachedFromActivityForConfigChanges() {
+        // Activity will be reattached, keep the reference
     }
 
-    override fun onDetachedFromActivityForConfigChanges() {}
     override fun onReattachedToActivityForConfigChanges(binding: ActivityPluginBinding) {
         activity = binding.activity
-        permissionHandler = PermissionHandler(context, binding.activity, permissionRequestCode)
+        permissionHandler?.attachActivity(binding.activity)
     }
 
     override fun onRequestPermissionsResult(