Revert "Revert "Make the network stack a persistent process.""

This reverts commit f6315c2e.

Reason for revert: Change I346d772e1f4aed94f6faead3b6455efc4666b651 implemented a feature to allow upgrading persistent processes via staged installs.

Bug: 131046856
Test: - adb install networkstack.apk fails
      - adb install --staged networkstack.apk succeds (after reboot)
Change-Id: I13af35c953c0501f2b129092e0089d4e893b97a1