system_server: allow handling app generated unix_stream_sockets

Allow system server to handle already open app unix_stream_sockets.
This is needed to support system_server receiving a socket
created using socketpair(AF_UNIX, SOCK_STREAM) and
socketpair(AF_UNIX, SOCK_SEQPACKET). Needed for future Android
functionality.

Addresses the following denial:

  type=1400 audit(0.0:9): avc: denied { read write } for path="socket:[14911]" dev="sockfs" ino=14911 scontext=u:r:system_server:s0 tcontext=u:r:platform_app:s0:c512,c768 tclass=unix_stream_socket permissive=0

Bug: 19648474
Change-Id: I4644e318aa74ada4d98b7f49a41d13a9b9584f39