Make the network stack a persistent process.

The network stack should never be killed, because if it does,
recovering networking requires a reboot. Make the process
persistent to reduce the chances of it being killed by the OOM
killer.

Bug: 131046856
Test: builds, boots
Test: process oom_score_adj is -800
Test: process goes from "Persistent Service" to "Persistent" in dumpsys meminfo
Change-Id: Ic8ebf6356e23489f6cd984d680ff9f1b4cbbe52d