System: Close log sockets prior to a fork.

Calling __android_log_close sometimes isn't sufficient because
some libraries statically link in liblog. This may not be a complete
or a clean change, but it's sufficient to get whitelisting working
on Lollipop and Marshmallow.

bug: 30963384
Change-Id: I38cdfc17e9d942417ca57154776bd28e5b78b807