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
diff --git a/libs/binder/trusty/OS.cpp b/libs/binder/trusty/OS.cpp
index 46346bb..1f9389a 100644
--- a/libs/binder/trusty/OS.cpp
+++ b/libs/binder/trusty/OS.cpp
@@ -52,4 +52,18 @@
     return RpcTransportCtxFactoryTipcTrusty::make();
 }
 
+int sendMessageOnSocket(
+        const RpcTransportFd& /* socket */, iovec* /* iovs */, int /* niovs */,
+        const std::vector<std::variant<base::unique_fd, base::borrowed_fd>>* /* ancillaryFds */) {
+    errno = ENOTSUP;
+    return -1;
+}
+
+int receiveMessageFromSocket(
+        const RpcTransportFd& /* socket */, iovec* /* iovs */, int /* niovs */,
+        std::vector<std::variant<base::unique_fd, base::borrowed_fd>>* /* ancillaryFds */) {
+    errno = ENOTSUP;
+    return -1;
+}
+
 } // namespace android