Add system_server to net_admin group

This allows system_server to search/read skfilter BPF programs. Skfilter
BPF programs status were previously dumped by Netd. In Android T, the
related code are mainlined and dumped by system_server process.
system_server needs to be in net_admin group so that it can read program
status.

$ adb root; adb shell ls -l /sys/fs/bpf/
-r--r----- 1 root net_admin ... prog_netd_skfilter_allowlist_xtbpf
-r--r----- 1 root net_admin ... prog_netd_skfilter_denylist_xtbpf
-r--r----- 1 root net_admin ... prog_netd_skfilter_egress_xtbpf
-r--r----- 1 root net_admin ... prog_netd_skfilter_ingress_xtbpf

Bug: 202086915
Test: test in Ib0e935ee2b714ac61daceba6d13fa7a20f97f68f
Change-Id: I8c48230a5da6873eee7d0ba183cb83e1d92cd8f6