Use flags = 0 to avoid fd closing for child updater process

If we use the default parameter we'll get O_CLOEXEC, which will close the fd
that we pass to the child process, which will cause it to fail. Passing zero
avoids the problem.

Bug: 122813742
Test: Verify that non-A/B OTA update is successful.
Change-Id: Ia61bc7260f17d9209715583e6ded69e1196ed3f6