[NS06] Implement the don't-reap mechanism

This exposes a mechanism for network providers to tell
the network stack that a given network must be kept up
for some specific reason. This is meant to be easier
for them than to have to file a request, in particular
because there is no guaranteed way to make sure the
request will be best matched by any given network.

Test: new test for this
Bug: 167544279
Change-Id: I238a3ee5ee9262477a23b897e4141769dd1505d1