Allow system_server to bind ping sockets.

This allows NetworkDiagnostics to send ping packets from specific
source addresses in order to detect reachability problems on the
reverse path.

This addresses the following denial:

[  209.744636] type=1400 audit(1441805730.510:14): avc: denied { node_bind } for pid=8347 comm="Thread-202" saddr=2400:xxxx:xxxx:xxxx:40b1:7e:a1d7:b3ae scontext=u:r:system_server:s0 tcontext=u:object_r:node:s0 tclass=rawip_socket permissive=0

Bug: 23661687
Change-Id: Ia93c14bc7fec17e2622e1b48bfbf591029d84be2