Fix a bug where listen callbacks would not be called

NetworkAgentInfos cache the list of requests they satisfy,
and that list is used to send callbacks. Therefore, when
the TRACK_DEFAULTs are copied, this list needs to be
updated.

The best way to do this is to figure out what was the old
active request and find which requests corresponds to it
in the new list, and then upon registering adding the
active request to the relevant satisfier if present.

A few other ways can be considered like replacing the
request as it gets added, but this would temporarily
increase the number of callbacks allocated to the app
and risks crashing it for no good reason ; furthermore
the call to remove would have to be eschewed somehow
for those requests that are replaced. This is much
simpler.

Test: new test for this. This also passes the future
      tests for per-profile default network preference.
Change-Id: I001351e5c478c2c77cbf2844abca77b205291778