gnetworkmonitornetlink: Fix potential GMainContext issue

Previously, the GSource would be attached to whatever GMainContext was
the thread default at the time; but that might no longer be the same as
the default at the time of constructing the GNetworkMonitor.

Save the default from construction time, so that source callbacks are
always invoked in the same GMainContext.

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

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