fastboot: socket testing improvements.

(This code was originally part of a huge fastboot CL but has been split
out to try to make the CLs a little more manageable).

More prep for fastboot TCP and UDP implementations. This CL adds a
SocketMock class that makes it easy to mock out network behavior so we
can unit test the TCP and UDP protocols.

Also uses the new libcutils socket_get_local_port() to avoid hardcoding
a server port in unit tests.

Bug: http://b/26157893.
Change-Id: I1ba10f31e98d7349313fc15f240383d63378a8db