Improved file descriptor cleanup in Zygote.

Enabling the blastula pool exercised a previously unused codepath that
wrote to the log.  This caused a socket to be opened when it wasn't
expected to be.  This patch re-closes the logging socket before applying
the SELinux policy for the new application.

This patch also adds code to clean up blastula table entries in
processes spawned from the Zygote and disables the blastula pool code
for child zygotes.

Bug: 123409530
Test: make & flash & boot & check logs for SELinux denials
Change-Id: I560cdf9c42502574d25ab25a0f8afa3eb6de307f