Make translate_packet take a fd instead of a tun header.

This will make it easier to use separate fds for reading and
writing in a future change.

(cherry picked from commit 91d0f1bc6dd24e54ed3caef9b08525b332ab0adf)

Bug: 15340961
Change-Id: I374b85f28ae570dc82d21e1ea82a2f072fd7fba3
4 files changed