Don't wait for message queues to become idle in wait-for-barrier.

It would take a long time for some of the message queues in
system_server process to become idle and we only need to make
sure any messages currently waiting have been handled as part of
"wait-for-barrier".

Bug: 260158381
Test: TH
Change-Id: Ibcb36ebf767806c23f087ed34ad8792672fbe0d6