Zygote: Unblock SIGCHLD in the parent after fork.

Follow up to change e9a525829a354c92983a. Allows the zygote to
receive SIGCHLD again and prevents the zygote from getting into a
zombie state if it's killed.

Change-Id: I81fde39a2dd32eaf7d8b397672d3165a4eb945e1
Contributed-By: rhed_jao <rhed_jao@htc.com>
Bug: 32693692
Test: manual
1 file changed