Make netd restart when system server hangs on it.

netd has been seen hanging (cause suspected but unconfirmed)
which then brings down the system server which blocks forever
on its main thread trying to connect to netd with its hung
event loop.

This then results in an infinite watchdog runtime restart loop
because the situation never improves:  netd is still borked.

Instead, we should kill netd and give it another chance when
the system server dies.

Bug: 3018996
Bug: 3017876
Change-Id: Ibf9ecf0ffcb78e48c38e5167f21cbeefe0e2a47f