Although, at the moment, we do not yet require an event bus to be present, as
we start to add more pieces we will want to ensure it is always available. Add
a new constructor and replace existing uses of new(tsd.System) throughout.
Update generated files for import changes.
Updates #15160
Change-Id: Ie5460985571ade87b8eac8b416948c7f49f0f64b
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>