libpdx_uds: Allow to create Endpoint/ClientChannel from a socket pair

This is important to enable Service/Client operation in unit tests.
Being able to create a pair of Unix domain sockets and construct both
Service and Client so that they can talk to each other without having
to create a physical socket file is convenient.

This change makes it possible to create an instance of Endpoint and
ClientChannel classes based just on a pair of sockets (Endpoint does
take another socket to simulate the main endpoint FD to accept incoming
connection on, but it is not used for this, only the shutdown events
are received from that main socket. Endpoint uses the channel FD to
perform actual communication with the client).

Bug: 37443070
Test: `libpdx_uds_tests` pass
Change-Id: Ifa1a9d03b97bd90282a04715c2105ad37a8de936