UsbTransportSniffer: Defensively use 'final'

UsbTransportSniffer invokes the virtual method Close() within
its destructor.  This usage is fine right now, but if a future
class inherited from UsbTransportSniffer and overrode the Close()
method, then this destructor would not act as expected.

To protect against this, we declare Close() 'final' in
UsbTransportSniffer, and add a comment.

Test: Treehugger
Change-Id: I6182d84e7caf67cb06f37c22c6203ad6d6e57f34