RpcBinder: Add AF_UNIX socketpair transport

Add support for running RpcBinder over unnamed Unix domain sockets
created by socketpair(). This is useful e.g. between parent/child
processes.

The implementation uses the initial socket pair only to create more
socket pairs for individual connections. This creates a natural mapping
to syscalls used on sockets bound to an address:

    socket()                socketpair()
    bind()                  n/a (preconnected)
    connect()               sendmsg()
    listen()                recvmsg()

Bug: 250685929
Test: atest binderRpcTest
Change-Id: Id4ff3946ddcfefe3592eb1149c61582f7369aa29