binderRpcTest: use waitpid

Actually reap child processes. This gives us stronger guarantees (that
everything can shut down) and it avoids 'kill'.

Bug: 186661301
Test: binderRpcTest
Change-Id: If10f00de845eb8097813b4edbf8e2b8ffdc90c5f
diff --git a/libs/binder/RpcSession.cpp b/libs/binder/RpcSession.cpp
index d5ce324..ac7544f 100644
--- a/libs/binder/RpcSession.cpp
+++ b/libs/binder/RpcSession.cpp
@@ -199,7 +199,8 @@
                 state()->getAndExecuteCommand(connection->fd, sp<RpcSession>::fromExisting(this));
 
         if (error != OK) {
-            ALOGI("Binder connection thread closing w/ status %s", statusToString(error).c_str());
+            LOG_RPC_DETAIL("Binder connection thread closing w/ status %s",
+                           statusToString(error).c_str());
             break;
         }
     }