Label /dev/socket/zygote_secondary

zygote_secondary talks over a different socket named
/dev/socket/zygote_secondary. Make sure it's properly labeled.

See https://android-review.googlesource.com/89604

Addresses the following denial:

<12>[   48.442004] type=1400 audit(1400801842.179:5): avc:  denied  { write } for  pid=1082 comm="main" name="zygote_secondary" dev="tmpfs" ino=9953 scontext=u:r:system_server:s0 tcontext=u:object_r:socket_device:s0 tclass=sock_file permissive=1

Bug: 13647418
Change-Id: I1ff5f1d614295a5870bb8a3992ad9167e1656c92