sepolicy: allow vpn connection via L2TP.

avc: denied { read write } for comm="pppd" path="socket:[44031]"
dev="sockfs" ino=44031 scontext=u:r:ppp:s0 tcontext=u:r:mtp:s0
tclass=unix_stream_socket.

CRs-fixed: 884186
Change-Id: I6eaacbcaeb9f70c2e9f774002547f8826b089844