Immediately cancel any pending Bluetooth operations.
The BluetoothSocket.connect() function blocks while waiting for a
connection, but it's thread safe and we can cancel that by calling
close() on it at any time.
Importantly we don't want to close the socket any other time, because we
want to leave that up to the user of the socket - if you call close()
twice, or close Input/Output streams associated with the socket
simultaneously, it can cause a segfault due to a bug in some Android
Bluetooth stacks. Awesome!