Reenable fdsan in the zygote post-fork.

By default, fdsan is disabled after forking to avoid breaking 'fork();
close(0..1024); exec(...)'. Reenable it in the zygote manually.

Bug: http://b/110100358
Test: booted walleye-aosp
Change-Id: I8706e4153e4e91306f50d215a62fe02d445082e1