Fix crash caused by USB device disconnection during permission grant - #86

Summary

• Fix a race condition crash (~5% repro rate) when rapidly plugging/unplugging USB devices. The permission dialog may grant access to a device that has already been physically disconnected, causing UsbManager.openDevice() to return null and getSerialNumber() to throw an uncaught SecurityException, killing the process.
• Add null-checks and exception guards at four critical points in USBMonitor and adapt the Flutter plugin caller accordingly.

Root Cause

When a USB device is unplugged while the system permission dialog is still showing, the following race occurs:

T1: Device attached → requestPermission() → system dialog shown
T2: Device physically unplugged → /dev/bus/usb/xxx removed
T3: User taps "Allow" → BroadcastReceiver gets EXTRA_PERMISSION_GRANTED=true
T4: processConnect() posts to mAsyncHandler (async delay widens the race window)
T5: UsbControlBlock constructor → openDevice() returns null (device gone)
T6: updateDeviceInfo() → getSerialNumber() → SecurityException → FATAL

Changes