feat: add Android USB serial port support via USB Host API

- Add USB Host permissions and device filter to AndroidManifest.xml
- Create UsbSerialPlugin Kotlin plugin for USB Host API (enumerate, permission, open devices)
- Add serial_connect_fd command for Android to accept USB file descriptors
- Create RawFdStream wrapper for async I/O on raw file descriptors
- Make run_serial_with_poll generic over AsyncRead+AsyncWrite
- Register UsbSerialPlugin in MainActivity

src-tauri/plugins/usb-serial-plugin.json

@@ -0,0 +1,9 @@
+{
+  "plugins": {
+    "usb-serial": {
+      "android": {
+        "package": "com.lenn.tauri_serial.UsbSerialPlugin"
+      }
+    }
+  }
+}