Fix#10588
Regression since #10305
While we're there, rejig the FL toggle callback shenanigans so that implementation details don't leak through to *other* implementations.
(i.e., leave the Kobo mess in Kobo land, with only a minimal impact on the public API and its implementation).
functionBasePowerD:turnOnFrontlightHW(done_callback)self:setIntensityHW(self.fl_intensity)end--- @fixme: what if fl_intensity == fl_min (c.f., kindle)?
--- @note: done_callback is used to display Notifications,
--- some implementations *may* need to handle it themselves because of timing constraints,
--- in which case they should return *true* here, so that the public API knows not to consume the callback early.