Add MANAGE_TEST_NETWORKS to system API

The system and particularly mainline modules need to reference the API
constant so they can verify that callers have the permission.

Bug: 174436414
Test: m
Merged-In: I9f41f888fbd4b7605d750110b345005a5cba85d1

Change-Id: I9f41f888fbd4b7605d750110b345005a5cba85d1