rpc_binder: Properly shut down on socketpair() EOF

FdTrigger::triggerablePoll returns OK on POLLIN (higher priority)
and DEAD_OBJECT on POLLHUP (lower priority). Unlike INET or vsock,
socketpair() sockets trigger POLLIN|POLLHUP on EOF and EOF is detected
by recvmsg() returning zero bytes.

Adjust the server accept loop to take this into account and correctly
break out of the loop.

Bug: 250685929
Test: cleanly shut down UDS bootstrap RpcServer
Change-Id: I56d6a67373f16b5b1bdc167de94a7252bedca8e2