libcutils: add socket_get_local_port().

Tests that require a local server currently hardcode a test value,
which can run into conflicts depending on what's currently running on
the machine.

This CL adds socket_get_local_port(), which lets us pass 0 so the
system picks an open port and we can query which port it chose.

Bug: http://b/26236380
Change-Id: I01d1558884e7636081d9a357db6faa86929934f6