test_vendor_lib: Implement event loop with libbase

Adds event loop functionality by handling IO in the Watcher class,
HciTransport. Also fixes a bug where the vendor's file descriptor was
closed early and another where an object was mistakingly being passed
by value.

Bug: 21586676
Change-Id: I2ec30b68c0ede47bf1ad78ec13beee93d0f8ab9d