Do not mark in-process NetworkStack as persistent

The in-process NetworkStack runs in the system process, so it does not
need to be marked persistent.
Doing so causes a spurious com.android.networkstack.inprocess process to
be running, wasting memory.

Bug: 131046856
Test: Flashed sailfish, networking works, no extra process started
Test: Flashed walleye, networking still works
Test: aapt dump xmltree NetworkStack.apk AndroidManifest.xml unchanged
Change-Id: I0eb896c825ffe2581950c0bdd2b31ebd7eae0982