breakManager: Update manager state after settings change

breakManager: Update manager state after settings change

The change of settings may have affected when a break is due. In
particular, `this._breakLastEnd` is modified in `_updateSettings()`,
but `this._state` is not modified correspondingly to transition to/from
a `BREAK_DUE` state (if appropriate).

This could be the cause of the `breakDueAgo should be non-negative`
assertion failure from #8280.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8280
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3687>