Don't refcount TRACK_DEFAULT requests.

This has no concrete impact on the behavior of ConnectivityService,
but in principle TRACK_DEFAULT requests should not be counted toward
requests that make a network foreground. It does not have an impact
because only VPNs could be affected by this, and VPNs are always in
the foreground by definition.

Test: runtest frameworks-net
Test: cts
Change-Id: Id2ae6b5c9d542fe168e64ed713b6ec0a04062c82