netd: reduce privileges

netd doesn't need full root capabilities. Rather, it only needs
CAP_NET_ADMIN and CAP_NET_RAW. Reduce the capabilities to that
set.

netd continues to run with UID=0, which allows applications spawned
by netd to continue to have CAP_NET_ADMIN and CAP_NET_RAW. It also
allows netd to access /proc and /sys files as UID=0.

Change-Id: I439d22150109697213c0cc83276ddb668007b978