Do not leak usb device connection FD

The native code uses mNativeContext=0 to indicate that is was already
closed and checks this properly. Hence let's leave the checking in the
native code.

We need to keep a reference in UsbRequest as otherwise the
UsbDeviceConnection might get finalized while a UsbRequest is in
progress. The UsbRequest itself makes sure that it is not garbage
collected while the I/O is in progress.

Also I added CloseGuards to make sure the classes are used properly and
fixed an error string in the native code.

Fixes: 31124312
Bug: 31021315
Change-Id: I96deb73957eba0e14e6b656988a2ae9b409bf55f