Mark NetworkStack as coreApp

Packages marked as coreApp will be included in the parsed packages when
booting in encrypted or encrypting state, on legacy devices (upgrading
from P) that support FDE, not FBE.

This fixes a bug on such devices where the system server cannot load
because of the missing NetworkStack.

Bug: 197282772
Test: Installed updated app on device, booted, checked connectivity,
      verified that PackageInfo has coreApp=true.
Change-Id: I0887b14819654f2c14501f78bfe5bc61d062dcf9