fastboot: fix TCP protocol version check.

Currently the TCP handshake fails if the device TCP protocol version
doesn't match the host exactly, but the protocol is supposed to allow
for forwards compatibility by accepting any protocol version >= itself.
That way the other side can potentially lower its protocol to match and
keep going.

This CL fixes the protocol version check and adds corresponding unit
tests.

Bug: http://b/27220700
Change-Id: Ib17f0a55eb910105a27609bc94bf76a30442e92e