Crash the system server if network stack is lost

Crash the system server if it cannot bind to the network stack, or if
the network stack crashes. This is better than keeping the system
running in a bad state where all networking would be broken.
Before the network stack was moved to a different process, the whole
system would go down when it encountered a crash.

Test: added crash in NetworkStack, verified system_server restart
Bug: 130028724
Change-Id: Ia8ee7699c4b15ba4b1116c7ba9171bf6034b52c3