Zygote : Block SIGCHLD during fork.

We close the android logging related sockets prior as late as possible
before every fork to avoid having to whitelist them. If one of the
zygote's children dies after this point (but prior to the fork), we can
end up reopening the logging sockets from the SIGCHLD signal handler.

To prevent this from happening, block SIGCHLD during this critical
section.

Bug: 32693692
Test: Manual

(cherry picked from commit e9a52582)

Zygote: Unblock SIGCHLD in the parent after fork.

Follow up to change e9a52582. Allows the zygote to
receive SIGCHLD again and prevents the zygote from getting into a
zombie state if it's killed.

Contributed-By: rhed_jao <rhed_jao@htc.com>
Bug: 32693692
Test: manual

(cherry picked from commit c7161f756e86b98f2244a04d9207b47149965fd7)

Change-Id: If89903a29c84dfc9b056f9e19618046874bba689
(cherry picked from commit dfcc79ee)