Ensure property_set connection sockets are CLOEXEC.

When handling a property control message to start a service, the
connection socket was being left open. The child socket FD would then be
inherited by the service from init as "/null".

Bug: 34698883

Test: adb shell start webview_zygote32; adb shell ls -l /proc/<pid>/fd,
      no "/null" FD open.

Change-Id: I25f34e1bfc1414e6143acf23414b05f5a241cace