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/tests/binderRpcTestCommon.h b/libs/binder/tests/binderRpcTestCommon.h
index 4513d36..712f37e 100644
--- a/libs/binder/tests/binderRpcTestCommon.h
+++ b/libs/binder/tests/binderRpcTestCommon.h
@@ -49,6 +49,7 @@
 
 #include "../BuildFlags.h"
 #include "../FdTrigger.h"
+#include "../OS.h"               // for testing UnixBootstrap clients
 #include "../RpcSocketAddress.h" // for testing preconnected clients
 #include "../RpcState.h"         // for debugging
 #include "../vm_sockets.h"       // for VMADDR_*
@@ -67,15 +68,19 @@
 enum class SocketType {
     PRECONNECTED,
     UNIX,
+    UNIX_BOOTSTRAP,
     VSOCK,
     INET,
 };
+
 static inline std::string PrintToString(SocketType socketType) {
     switch (socketType) {
         case SocketType::PRECONNECTED:
             return "preconnected_uds";
         case SocketType::UNIX:
             return "unix_domain_socket";
+        case SocketType::UNIX_BOOTSTRAP:
+            return "unix_domain_socket_bootstrap";
         case SocketType::VSOCK:
             return "vm_socket";
         case SocketType::INET: