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/tests/IBinderRpcTest.aidl b/libs/binder/tests/IBinderRpcTest.aidl
index 41daccc..646bcc6 100644
--- a/libs/binder/tests/IBinderRpcTest.aidl
+++ b/libs/binder/tests/IBinderRpcTest.aidl
@@ -55,6 +55,7 @@
     oneway void sleepMsAsync(int ms);
 
     void die(boolean cleanup);
+    void scheduleShutdown();
 
     void useKernelBinderCallingId();
 }