Only disable async preemption in debug mode.

Colin and Jin opined that we shouldn't change production in order to
make debugging easier. I originally thought it's better to have one
fewer knob but I don't have very strong feelings about it and their line
of reasoning makes total sense.

Test: Presubmits.
Change-Id: Id8a914aebbeffec6e8f860157c13f81093194f14