Handled Null Exceptions

[Jerinji2016]

Changes

1. Changed USBPrinterAdapter to singleton class.

Previously, USBPrinterAdapter().getInstance() worked same as constructor instantiating.
Changed getInstance to static method and disposed the static instance on onDetachedFromEngine

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2. mUsbDeviceReceiver receiver always receives null for usbDevice

Checked in Android 12 & 13, the intent extra for UsbManager.EXTRA_DEVICE is always null.
So, add the vendorId and productId to pendingIntent while requesting permissions.
If UsbManager.EXTRA_DEVICE is null, then try to init usb device with vendorId and productId.

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3. Moved pendingIntent from class scope to local scope

Since pending intent is only required when requesting permission, moved variable to local scope