ServiceManager: Implement PING_TRANSACTION

Stop printing "invalid id " to stderr every time a process tries to
connect to the servicemanager.

Change-Id: Ib0e5a0375bfa2dec2c2f9cd668bd5dda46ed6588
diff --git a/cmds/servicemanager/service_manager.c b/cmds/servicemanager/service_manager.c
index f8212e8..79ce6ed 100644
--- a/cmds/servicemanager/service_manager.c
+++ b/cmds/servicemanager/service_manager.c
@@ -220,6 +220,9 @@
     if (txn->target.handle != svcmgr_handle)
         return -1;
 
+    if (txn->code == PING_TRANSACTION)
+        return 0;
+
     // Equivalent to Parcel::enforceInterface(), reading the RPC
     // header with the strict mode policy mask and the interface name.
     // Note that we ignore the strict_policy and don't propagate it