Remove legacy keep_alive.c; now require libusb >= 1.0.18

Signed-off-by: Benn Snyder <benn.snyder@gmail.com>
Use audio serial as a fallback for K4W and 1473 models that do not provide a useful camera serial (thanks @olzhas).
Fixes #360 and resolves #393.

Signed-off-by: Benn Snyder <benn.snyder@gmail.com>