Add support for preconnected RPC client

Normally we want to call RpcClient directly, but the apps don't have
access to create a client vsock themselves. This change adds a helper
function to establish an RPC session with preconnected file descriptor.

Bug: 195381416
Test: atest MicrodroidHostTestCases
Change-Id: Idbdc6b00ca459ed10b1c79e64f004708594b92da
diff --git a/libs/binder/libbinder_rpc_unstable.map.txt b/libs/binder/libbinder_rpc_unstable.map.txt
index 3921a4d..1138786 100644
--- a/libs/binder/libbinder_rpc_unstable.map.txt
+++ b/libs/binder/libbinder_rpc_unstable.map.txt
@@ -2,6 +2,7 @@
   global:
     RunRpcServer;
     RpcClient;
+    RpcPreconnectedClient;
   local:
     *;
 };