Fix crash on startup for older Android devices

The overload to the registerReceiver method with a flags parameter was
introduced in API level 26 (Android 8.0), which leads to an instant
crash on startup on devices running older versions of Android. Adjusted
the implementation to use the previous signature of registerReceiver on
devices with API levels below 26.

android/src/org/mavlink/qgroundcontrol/QGCUsbSerialManager.java

@@ -106,13 +106,15 @@ private static void registerUsbReceiver(Context context) {
         filter.addAction(BluetoothDevice.ACTION_ACL_CONNECTED);
         filter.addAction(BluetoothDevice.ACTION_ACL_DISCONNECTED);
 
-        int flags = 0;
-        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.TIRAMISU) {
-            flags = Context.RECEIVER_NOT_EXPORTED;
-        }
-
         try {
-            context.registerReceiver(usbReceiver, filter, flags);
+            if (android.os.Build.VERSION.SDK_INT >=
+                android.os.Build.VERSION_CODES.TIRAMISU) {
+                int flags = Context.RECEIVER_NOT_EXPORTED;
+                context.registerReceiver(usbReceiver, filter, flags);
+            } else {
+                context.registerReceiver(usbReceiver, filter);
+            }
+
             QGCLogger.i(TAG, "BroadcastReceiver registered successfully.");
         } catch (Exception e) {
             QGCLogger.e(TAG, "Failed to register BroadcastReceiver", e);