gnetworkmonitornetlink: Use a coarser-grained timer for dumps

By using g_timeout_source_new_seconds(), we can let timer wakeups be
coalesced by the scheduler, and reduce power consumption a bit. This
shouldn’t really affect the accuracy of the network monitoring.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=793880