libbinder: reverse connections

When connecting to an RPC client server, you can request to serve a
threadpool so that you can receive callbacks from it.

Future considerations:
- starting threads dynamically (likely very, very soon after this CL)
- combining threadpools (as needed)

Bug: 185167543
Test: binderRpcTest
Change-Id: I992959e963ebc1b3da2f89fdb6c1ec625cb51af4
diff --git a/libs/binder/RpcState.cpp b/libs/binder/RpcState.cpp
index 2cad2ae..2f6b1b3 100644
--- a/libs/binder/RpcState.cpp
+++ b/libs/binder/RpcState.cpp
@@ -383,6 +383,7 @@
         return status;
 
     if (flags & IBinder::FLAG_ONEWAY) {
+        LOG_RPC_DETAIL("Oneway command, so no longer waiting on %d", fd.get());
         return OK; // do not wait for result
     }