Use backstop timeouts on asynchronous countdown during preflight

Work around nebulous lost-timeout issues by adding a backstop timeout
to "wait for result" latch operations.  When we hit these, the initial
conditions will be reported as final result; so make those intial states
match the error condition that is appropriate to such a timeout.

Bug 28963707

Change-Id: I4d21a86c48e87633118b1e6eaa05c1d966efec81