Support ADB over vsock from the guest and the host.

The implementation is based off of socket_forward_proxy, but the data is
sent through a vsock file descriptor instead of the shared memory
window.

This allows replacing one of the dependencies on the shared memory
window with a dependency on vsock.

Test: adb bugreport on launch_cvd -kernel=...cf-4.14... -vsock_guest_cid=3 -adb_mode=vsock_tunnel
Bug: 121166534
Change-Id: I5bde24efa2c712c2e67fe8615c2e77ec9164e182