adbd: implement a nonblocking USB Connection.

Implement a Connection that implements a nonblocking interface to
functionfs, to replace the existing implementation that uses two
threads that loop and call read and write respectively. The existing
implementation is vulnerable to a race condition that can occur when a
connection is terminated, where one thread can notice failure and
complete reinitialization of the USB endpoints before the other thread
noticed anything went wrong, resulting in either the first packet
coming from the other end disappearing in to the void, or the other end
getting a packet of garbage.

As a side benefit, this improves performance on walleye from:

    push 100MiB: 10 runs: median 49.48 MiB/s, mean 50.00 MiB/s, stddev: 2.77 MiB/s
    pull 100MiB: 10 runs: median 75.82 MiB/s, mean 76.18 MiB/s, stddev: 6.60 MiB/s

to:

    push 100MiB: 10 runs: median 73.90 MiB/s, mean 73.51 MiB/s, stddev: 5.26 MiB/s
    pull 100MiB: 10 runs: median 105.90 MiB/s, mean 107.19 MiB/s, stddev: 6.10 MiB/s

Test: python test_device.py
Change-Id: I9b77c1057965edfef739ed9736e5d76613adf60a