Ensure that the zygote is single-threaded before forking.

We really need to wait for the pthreads to exit.

Bug: 4235038

git cherry-pick 9aa53337a20c6da1e026bc1b06cd3930db49349a

Change-Id: I0225530dca7d90bdf972ace096fc21901950c1e3
1 file changed